Loads of great perspectives, good advice on listening back to recordings together, maybe an offer to help outside of rehearsal, and regardless, you’ve probably decided.
In this situation, and I want to play, it can only benefit from affirmative actions at the source of the difficulty. I’m sympathetic to old timers whose voice or playing aren’t what they used to be. (Cuz soon, I are one.) But if I don’t want to play, that’s my decision, and “hard to find a replacement” is not really my problem.
So on the vocal, my take is “garbage in, garbage out”. Pitch correction live with a major artist, and the best gear, and the best sounds pros…still relies on more than a bit of luck, and still might not sound that good.
And actually, the source of the challenge is the vocal instrument and the vocalist (who has had a good musical ear in the past, I’m assuming) being in the best condition possible, AND being able to hear themselves. So vocal practice, vocal coaching, good monitor set up/judicious use of in-ear or whatever technology, trying different keys. And until that is working, drop playing other instruments simultaneously with singing, and drop autocorrection because it distracts from fixing the actual problem. If the your LP had a cracked neck and missing tuning key, you wouldn’t replace your amp tubes and auto tune and call it good.
I’d offer make the personal investment to help the singer (thus, the band), and if that met resistance, I’d move on. In my current situation, I’m helping the singer learn songs outside rehearsal, which means rehearsal can be “rehearsal” not “basic personal song learning practice, waste of the band’s time”, and as a bonus, that keeps him off the trumpet.
