Prefixes such as saṃ have very general and vague meanings. The most common meaning for saṃ is “together”. For instance, saṃvāsa, meaning “living together”, is a combination of the stem vāsa, “live”, and the prefix saṃ. The exact same prefix is used in the same way in many Germanic languages, such as German and the Scandinavian languages, once again showing how culturally connected we all are.
But often saṃ has no discernible meaning. In these cases it is used more as a convention, perhaps because the word sounds better with a prefix, or some other reason not related to meaning. For instance, you find both cetanā and sañcetanā (= saṃ + cetanā) in exactly the same meaning, but used in different contexts.
With saṃvara, it seems the prefix has no particular meaning. The stem vara does not seem to be used by itself and so some prefix is presumably required.
Abhijjhā-domanassa occurs in the standard description of both sense restraint and satipaṭṭhāna. In the former case it is about avoiding the arising of these defilements, whereas in the latter case they have been overcome (vineyya loke abhijjhā-domanassa; where vineyya means “having removed”). So sense restraint is about holding the defilements at bay, whereas in satipaṭṭhāna they have been largely overcome. This is why sense restraint comes before satipaṭṭhāna in the gradual training.
I think it is fair to say - as the commentary to the Satipaṭṭhāna Sutta does - that this roughly refers to overcoming the five hindrances. The first two hindrances are the main culprits for the existence of the remaining three, which means that once they have been removed, then all five are largely gone. I do not think, however, this is meant in an absolute sense. After all, satipaṭṭhāna practice is about removing the last remainder of the five hindrances. Only after successful satipaṭṭhāna practice is the mind ready for samādhi proper.
I believe he too uses “restraint”. (I have just checked, and he does indeed use restraint.)
As I have implied above, I don’t really think this works, because saṃvara comes well before the abandoning of the five hindrances in the gradual training.
Spot on.
Yes, I would agree. It is perhaps a bit more gentle than “restraint”.
I am very pleased to be able to share. This is one of the wonderful benefits of having studied this in detail for over two decades. 
But personal reflection is so important. I am glad to see that you are considering all this so carefully. This, say the suttas, is a very important part of the path.
Well, yes, but it is also a matter of training the mind until it becomes more or less automatic. It is about shifting your perceptions in a relatively permanent fashion. You don’t get angry because you simply view people in a different way. But you are quite right, of course, that it takes persistence and commitment, but I wouldn’t say it is difficult.
Yes, clearly the lack of hair is a problem for us! Perhaps we need to resort to boot-straps instead? Or, in the absence of boots, thong-straps? With a bit of redesigning, we could admonish each other: “Pull yourself up by your thong-straps, Venerable!” Hmm … not sure. But I do like your quaint sense of humour. 
I agree that restrain inevitably will involve a degree of will-power. It’s so hard-wired into us that we can hardly avoid it. I think people often use will-power when their mental content does not match their self-perception. “I shouldn’t think that …”, suppress. It is just so automatic. And this is precisely why I believe the wisdom faculty needs to be emphasised. If we are not told about the alternative, then chances are we will just continue with our old habits. I believe the most profitable course is to put our attention on developing the wisdom/reflective side, and just allow the will-power to work according to habit. In other words, we don’t need to invest in the will-power side; it’s already strong enough, arising as it does from the sense of self. Gradually the balance will change and proper reflection will become our main tool in dealing with defilements.
