When an editor asks you to revise a paper taking reviewers’ comments into account, they mean just that — they do not mean “follow all the reviewers’ suggestions precisely”. In my experience as an author, on any issues bigger than typos, it’s fairly usual that I (and/or co-authors) don’t quite agree with a referee’s suggested edits, but their comment helps me understand what was sub-optimal about my/our original version, and so find an alternative way to improve that shortcoming. And when I’m a referee, reading revisions, I never expect authors to have followed my suggestions to the letter — just to have considered my concerns and addressed them in some reasonable way.
So in cases like yours, the kind of things which might help allay both referees’ concerns are:
Start the proof by explicitly signposting/acknowledging what is easy about it. “This proof will be routine for experts, as a fairly standard application of pseudolinear estimation techniques.” This allays two possible sources of Referee B’s unhappiness: it lets them know that they can safely skip the proof, and it makes clear that you’re not overstating the novelty of this theorem.
Within the proof, be conscious which details are more difficult (bearing in mind Referee A’s comments), and explain those clearly; but also, to help highlighting them, don’t belabour the parts that really are routine. A step-by-step argument like
A = B (by the second unitary condition) ≤ C (by convexity) = D (by Kane’s lemma, part (iii)) < E (by Chen’s inequality).
can often be clearer as
It follows directly by unitarity, convexity of f, and Kane’s lemma that A ≤ D; then D < F by Chen’s inequality (with ε = 1/7, and the stochasticity hypothesis ensured by Lemma 3.2).
I don’t mean to suggest that those are necessarily applicable in your case — but rather to illustrate how even when the referees’ specific suggestions are incompatible, the concerns behind them may both be reasonable, and it can be possible to find a revision addressing them both at once.