Is the Noble Lie Acceptable?

Mental reservations should be used sparingly.  For one thing, if a non-Catholic "catches wise" to what Catholics are doing in this regard, they then have to parse every word we say, "are they telling the plain and whole truth, or are they speaking ambiguously to conceal the truth?".  So far as I am aware, mental reservation is solely a Catholic concept.  We are basically taking advantage of someone else's lack of mental agility.  I guess Protestants just go ahead, tell the lie, and throw themselves on the mercy of Our Lord's Passion, which in the minds of many of them, "takes care of" all sin, both past, present, and future.

It is not acceptable in court, in fact, oaths typically take the form of "the truth, the whole truth, and nothing but the truth", and some oaths specifically exclude mental reservation and similar concepts.  Where there is a grave reason to conceal or obscure some element of "the truth", to keep from incriminating oneself or another for a serious reason, I cannot say where you "draw that line".  Every situation would be different.

An outright lie is not a "mental reservation", regardless of how "noble" it is.

I had to use a mental reservation the other day, horrible as it might sound --- and if it sounds that way to you, stop and realize that you don't know the circumstances --- with my own dear mother, when she told me in no uncertain terms that she did not want me having any more Masses said for my father.  She does not comprehend the necessity of this, and sad to say, she still proceeds in some ways from Protestant error, she was never catechized properly.  (I am having her to sit in on my son's Baltimore Catechism classes, to try and correct this.)  I told her "I am not having any more Masses said right now", meaning in my mind, "this very moment".  If she knew that I had the thirty Gregorian Traditional Latin Masses celebrated --- the SSPX's stipend is $800 (ouch!) --- well, her reaction wouldn't be anything you'd want to see, that's all I can say.  (I had to take this out of my 401(k), because I don't just have $800 lying around to spare.)  My father's reaction, for much the same reasons, would have been similar, but now, in eternity, I know he can now see the value of it.  My need now to obtain plenary indulgences for him, has "forced" me into more frequent confession, and greater attempts to grow in holiness, because there is that pesky little matter of having to be free of attachment to all sin, even venial.  Pretty tricky for me.

WRT your first point, I'm really not sure how many of the more fundamentalist/evangelical type of Protestants handle sin and repentance.  First of all, some proceed from the idea that when they accept Christ, they will quit sinning.  I saw on a bumper sticker one time (always a reliable source of truth), and unless I dreamed it, I know I also heard a well-known TV evangelist say, "Christians don't sin".  Not sure if there was a deeper meaning of "once you accept Christ, all your sin belongs to Him, and it isn't yours anymore", or if they literally meant "Christians do not commit sinful acts".  When Jimmy Swaggart gave his tear-filled sermon on having fallen from grace with a prostitute, I'll give him this much, at least he confessed publicly that he had sinned, he owned what he had done, and didn't try to cover it up or sugar-coat it.  That alone gave me some respect for him.

Also, "Social respectability", i.e., not sinning or appearing to sin in public, is huge in their circles.  Secondly, if they believe in "once in grace, always in grace", even if they do commit a sinful act, my understanding is that they don't regard that sin as "theirs" anymore, rather, it was put up on the cross when Our Lord suffered and died, and when they "get saved", the slate is wiped clean, past, present, and future.

Secondly, Masses by themselves don't confer a plenary indulgence, and some souls have many Masses said for them, years after their death.  As far as those acts by us that are done in pursuit of a plenary indulgence, you have to be free from attachment to all sin, even venial sin, and since I am hesitant to proclaim that I am ever in that state, for lack of a better way to put it, I always "second-guess" my attempts at obtaining them, and leave the rest to the mercy of God.  I figure that even if my plenary "took", obtaining plenaries time and again can't hurt anything, if they are "more than is needed", Our Lord will always apply them to another soul in need.  Better to attempt plenaries knowing there may be an unknown or unwanted attachment to sin in oneself, than not to attempt them at all.  (And, yes, there have been times in my life when I've had to say "there's no use in even trying to obtain one, I can't go that far".)

It is indeed, in whatever form or rite as the case may be. 

But following your reasoning, that would mean that a single Mass, celebrated for the repose of a soul who died in the state of grace, would all by itself, be sufficient to liberate that soul from purgatory, and that no further Masses for that departed soul would be necessary.  If that were the case, then the whole concept of Gregorian Masses --- 30 consecutive Masses, not just one --- would be flawed.  And it would be incumbent upon the pastor of a parish to say "I can't accept more than one Mass stipend for any one soul, because once a single Mass has been said, that is sufficient".  Clearly this does not happen, as I pointed out, some people have many Masses said for them, for years after they've died, people keep giving stipends.

I think "infinite value" and "plenary indulgence" are getting conflated here.