I think without the factors of each rule, as well as a clear definition and the untranslated Pāli word of ‘willingly’ in Pārājika 1 and ‘lusting’ in Pārājika 5 and 8, it will be difficult to find conclusive answers to questions.
Clearly, consent is a crucial aspect of these three rules, and quite a complex one at that. If it can be of use, here is the section of knowledge and consent in Ṭhānissaro Bhikkhu’s Buddhist Monastic Code I (PDF), pp. 45-46:
Knowledge & consent. For sexual intercourse to count as an offense, the bhikkhu must know that it is happening and give his consent. Thus if he is sexually assaulted while asleep or otherwise unconscious and remains oblivious to what is happening, he incurs no penalty. If, however, he becomes conscious during the assault or was conscious right from the start, then whether he incurs a penalty depends on whether he gives his consent during any part of the act.
Strangely enough, neither the Canon nor the Commentary discusses the factor of consent in any detail, except to mention by way of passing that it can apply to the stage of inserting, being fully inserted, staying in place, or pulling out. From the examples in the Vinita-vatthu, it would appear that consent refers to a mental state of acquiescence, together with its physical or verbal expression. Mere physical compliance does not count, as there are cases where bhikkhus forced into intercourse comply physically but without consenting mentally and so are absolved of any offense. However, there is also a case in which a woman invites a bhikkhu to engage in sexual intercourse, saying that she will do all the work while he can avoid an offense by doing nothing. The bhikkhu does as she tells him to, but when the case comes to the Buddha’s attention, the Buddha imposes a pārājika on the act without even asking the bhikkhu whether he consented or not. The assumption is that complying with a request like this indicates consent, regardless of whether one makes any physical or verbal movement at all.
Taken together, these cases imply that if one is sexually assaulted, one is completely absolved from an offense only if (1) one does not give one’s mental consent at any time during the act or (2) one does feel mental consent during at least part of the act but puts up a struggle so as not to express that consent physically or verbally in any way. (As the Commentary notes, drawing a general principle from the Vinita-vatthu to Pr 2, mere mental consent without physical expression is not enough to count as a factor of an offense, for there is no offense simply in the arising of a thought or mental state.) If one puts up no struggle and feels mental consent, even if only fleetingly during the stage of inserting, being fully inserted, staying in place, or pulling out, one incurs the full penalty. This would seem to be the basis for the Commentary’s warning in its discussion of the Vinita-vatthu case in which a bhikkhu wakes up to find himself being sexually assaulted by a woman, gives her a kick, and sends her rolling. The warning: This is how a bhikkhu still subject to sensual lust should act if he wants to protect his state of mind.
The Vinita-vatthu contains a case in which a bhikkhu with “impaired faculties”—one who feels neither pleasure nor pain during intercourse—engages in intercourse under the assumption that his impairment exempts him from the rule. The case is brought to the Buddha, who states, “Whether this worthless man did or didn’t feel [anything], it is a case involving defeat.” From this ruling it can be argued that a bhikkhu indulging in intercourse as part of a tantric ritual incurs the full penalty even if he doesn’t feel pleasure in the course of the act.
Like a lot of the rules, the goal is to cover every possible circumstance. If you take as example such as the one in the first paragraph above, where a bhikkhunī were to be assaulted during sleep, if she were to gain consciousness during the act, it would fall under Pārājika 1 (having not consented to the actions described in Pārājika 5 and 8). The subsequent mental and physical actions of the individual after this (such as described in the third paragraph above) would determine as to whether the offence is a pārājika, one of the lesser offences or if it is considered an offence at all—so such a situation would not result in disrobing (although surely a very traumatic one).
Then there is the issue of which Ayya Vimalanyani mentioned about sexual acts with a woman—and that Pārājika 5 and 8 could be avoided by bhikkhinīs who are only attracted to individuals of the same sex ("Should any bhikkhuni, lusting, consent to a lusting man’s rubbing, […]).
Also, I am guessing that ‘sexual acts’ for bhikkhuinīs, in Pārājika 1, has it’s own explanation, whether it is female-male or female-female—and this possibly being why penetration wasn’t used as the determining criterion, such as Pārājika 1 for bhikkhus.
