Dear Nicola
What an interesting sutta! Great question and how good it is to read everyone’s wonderful responses. 
So my take is as follows:
Who knows what context this sutta was given in… I find the mention of ‘enemies’ intriguing; it makes me think that this sutta was in response to somebody’s specific and problematic situation that involved ‘enemies’.
I think the first two stanzas are pointing towards both the Path, and the realisation of the Path. I mean to be so unshaken, is to have achieved some stage of Awakening - ‘the realisation of the Path’. But the Path is also to be cultivated…thus it’s for those many of us who are still at this stage. So for me, it feels like frequent investigation in to what is stopping me from the ideal (of the Awakened) and what i should be doing about these obstacles…and I keep coming back to the Path, to my Practice.
I really appreciate how Robert has used the Arahant as inspiration. A beautiful way of using Sanghanussati or Recollection of the Sangha…or it could even be Dhammanussati cos you’re reflecting on the growth of the Dhamma in a human being? It’s a wonderful technique to cultivate.
While this is a marvellous and meaningful and useful technique - reminding me of the brief list of other skillful means in the the 3rd stanza - I think, speaking from my pesonal experience, one of the traps us Buddhists fall into, speaking for myself at any rate, is to aim for the ideal by trying to ‘be’ the ideal. Often, this doesn’t work very well because it bypasses the present moment and in bypassing the present moment, we are bypassing the truth of how things are for us, and in bypassing that, we bypass all chance for genuine peace, truth and understanding to grow within our own minds. We essentially stop the Dhamma becoming our Dhamma; it remains the Dhamma of the Buddha, but it never grows within us…because instead of looking for it within, we are reaching hopelessly for it out there, in the ideal.
I find, personally, it’s not easy to be deeply present…I find a strong resistance in my mind and a strong tendency to go ‘out there’ rather than remain ‘in here’. Sometimes, I feel that all that can be done is to be present to that too!! Actually, remembering Ajahn Brahm’s teachings on retreat… YouTube …this is probably not a bad technique to use and it links in with the idea of being ‘patient’ because this kind of patience can bear fruit; I mean, I can’t make myself stop resisting the moment, but I can put the causes in place for it to happen naturally.
Apologies…I’m rambling a bit…but perhaps, if I may say so…a nice little ramble and so I’ve not deleted it 
Regarding the 3rd stanza, i’m reminded of Ajahn Brahm saying, “if you can do something, do it; if you can’t, then do nothing.” I think when appropriate, the kamma of doing nothing, particularly whilst meditating, is the kamma that ends all kamma. I think this is why one often hears the quote, ‘patient endurance is the highest virtue’. However, getting back to the stanza, I think this stanza is focusing on those times when you can do something and is giving a few examples that perhaps might have been useful to those who may’ve been present when this sutta was first spoken. I mean, I was just listening to the Friday night talk and a lady in the audience said she finds bringing metta into chanting, whilst remember anger inducing situations, really helps her to respond better in future. And Ajahn Brahmali responded and part of what he said was that yes, you can use things like chanting in a meaningful way, as opposed to a purely ritualised way.
With regard to the last stanza, I’m reminded of the story Ajahn Brahm relates about the fellow running through the jungle, chased by his kamma (a tiger) who falls into a well and there’s a dangerous snake in it and as he’s falling he grabs a branch, just out of reach of both animals. As he hangs there he notices a white mouse and a black mouse (the days and nights that are relentlessly passing) slowly biting through the branch. Looking up he
sees a beehive dripping honey. The honey drops on to his tongue…he enjoys this taste.
Regarding your very good questions:
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“One’s gain”… hmm, I wonder if you are using this phrase in a different way to the sutta?
There’s “gain” as the opposite of renunciation - which is how I think you’re using it.
There’s “gain” as the succesful action taken to alleviate immediate and long term suffering - which is how I think it’s been used in this sutta.
The question about anticipation…etc… It’s a tricky one…perhaps it’s more to do with getting to know ourselves and our own unique Practices? Then perhaps one knows when one has tipped over into a lack of usefulness and over-thinking? I’m not sure…
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I’m wondering if perhaps this is something we can’t know and thus in some sense have a lack of control over…in the sense that to know something is to have some control over it…The whole “knowledge is power” thing. I mean sometimes, when I’m weeping and wailing, it might not be kamma that’s strong…rather my hindrances and kilesas are strong!! So perhaps, in attempting some answer, I’d say, just keep gently doing the best you can…
Also I’m noting the comments of others here, who are pointing out that it is with rebirth knowledge, that knowledge of kamma really becomes powerful…and then perhaps, genuiniely useful…because it won’t be something intellectual and ‘out there’…but unique to you, deeply present, and ‘in here’.
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Hmmm…‘right patience’… reminds me of Ajahn Brahm talking about ‘waiting in the moment’ versus ‘waiting on the moment’. The former being the one recommended as it’s more deeply present to whatever is going on. Perhaps that’s the key? The more we practice, the more we cultivate being present, the more likely we will be to know for ourselves, that now is the time to wait and now is the time to act.
Also…just to add…though I don’t think you’ve explicitly asked about this, and I think someone else has mentioned how in EBT kamma isn’t fatalism… Anyway…just to add that it shouldn’t matter whether a situation is kammic or not…What matters is how we deal with it now and what we do about it now. You know, personally, I’ve really appreciated having this spelled out. It really gives one a sense of power over one’s life…in a sense, our two good Ajahns are conditioning us to realise that we can, either mentally, verbally or physically, do something about whatever situation we’re in, regardless of whether it was caused by kamma or some other cause…such as bad weather! Lol…if only it were as easy to do as it is to say…
As an important aside, I don’t think anyone should ever allow themselves to suffer abuse by thinking that they deserve it (it’s my kamma cos I did this in the past to someone)…though having said this, I think self-view, ill will and other factors cause us to do just this! Whilst kamma may be involved, I think the kilesas of delusion and hatred are in operation here…they’ve created some kind of deep, destructive, guilt complex…which is only going to be a type of kamma that makes the situation worse and encourages this person to become enmeshed in further situations like this in the future. Perhaps, the reflection that this is only going to perpetuate the cycle and thus draw others in too, is motivation to pull ourselves out of this kind of self-destructive thinking…cos self-destruction doesn’t happen in a vacuum, others get hurt too…
Buddhism’s about getting out of suffering. We take the panadol so we can meditate. We love ourselves so we can meditate. We don’t allow ourselves to be harmed, so we can meditate… And the meditation itself is for peace, for love, for growth in wisdom…anytime taking action grows those things…it’s gotta be a good action…anytime being patient in the moment grows those things, then perhaps then, that’s the way to go too…
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Having said all this, may I also say, just the act of asking this question, in this forum, has created a cause, a condition, which will now most probably encourage yourself and those of us who’ve read your post, to question more deeply, in the moment: is there something I can do skillfully in this situation? Personally, I’m hoping this conditioning, which you’ve skillfully put into place Nicola, will come up when next I’m feeling squeezed by kilesas! Actually, perhaps even my writing this will reitierate to me, next time it happens, the importance of not getting sucked into these particular ways in which the kilesas manifest to torture me!!
Which leads me rather nicely…to say…thank you…to yourself for your questions and for pointing out this sutta and also to the others respondents for their unique and useful contributions.
Much Metta and thanks and I hope I’ve not just gone and confused things further!! 
- Just in case you were getting ideas! 
) but I might see myself sacrificing my life for my nieces or nephews. And I think if I should ever get myself into a war situation where I was one-on-one with the enemy, I would rather let myself be killed than kill another human. If there’s rebirth, great; if there’s no rebirth, even better (problem solved 
).
Everyone has given great answers to your questions so I’m just gonna make my addition short 