I have not gotten to Longford's book yet, but it's on my 'to do' because it is cited quite a lot in Dr. P's book. Taking in my post again and reading yours, I see that you 'hit the nail on the head' with your point about the advice coming from too small and questionable a source, and in pointing that out, said much more eloquently what I, with three times the word count, only scratched the surface of. I think so far Dr. Pimlott did a phenomenal job of this account of Her Majesty's reign, but did expect a 30-year-old woman, this fresh and wide-eyed Queen of only some five years, to feel up to making a choice like this without totally relying on whatever she was told? I realize that I don't have the kind of training she had even then, but speaking as someone who is now 30, I can say that I imagine in the same situation I would have done whatever the 'elders' told me to do on that decision. In fairness, Dr. P does concede that with old man Churchill giving his backing for Macmillan, there was surely no chance of HM going against the endorsement of so formidable a statesman, regardless of how the endorsements were being formed.
Re-reading part of this section of P's book, I see that his argument here is closer to what you said than to what I suggested before. It seems that (if I am understanding both him and you, finally), in the absence of Conservative Party protocol for assigning a Leader, they in theory had only to rely on the royal prerogative for choosing Eden's replacement. However, none of them wishing to have the Queen decide (unilaterally, as you say) on this matter, the Cabinet ministers huddled together, and when they had their preference decided (Macmillan) they procured Churchill (no doubt from a deep sleep) for his very critical endorsement. (What would be fascinating to know is if the Queen would have so willingly gone with Macmillan if Churchill had given his endorsement for Butler! Yet surely, as Pimlott did concede, Churchill was just going along with what had already been "decided", for what did he care in his retirement?)
Apparently (Pimlott cites Eden's memoirs here) the Queen "enabled" Eden to "unequivocally recommend", but not "constitutionally advise" Butler. (Pimlott p.259). So without the Cabinet ministers and Churchill making their endorsement known to the royal household, would the Queen have chosen Butler? It's a very interesting question. Going on your statement, I would say that she would have chosen Butler in such a hypothetical case, but is it possible that she would have sought another "unequivocal recommendation"?