So everyone here agrees they've never played a perfect night?
So there's no problem if there's a few mistakes on the fill in gig, you can learn the 90 tunes, recognizing it won't be perfect.
That sort of situation, the important part is knowing all the parts, and listening hard. Go to a chorus instead of a bridge, at least know the bridge chords, know a transition is coming up, and if you guess wrong, be quick about getting back on track.
Any cover band plays popular stuff. So you don't have to learn 90 songs from scratch, you've heard most of them, played a fair number of them. Assuming you have a good ear, you can fake your way through most of many of the ones on the list you haven't played.
Pop songs are pretty easy. You get a bunch of really talented musicians together, they'll make magic, play with a lot of energy, and will be quick enough on the draw, and listening to each other, such that any mistakes are covered very well.
I just joined a new band, had 7 rehearsals, 2 a week for a little over 3 weeks. Had to cover 57 tunes in that time. That's really too many rehearsals, a night spent rehearsing is better spent on your own working on the tunes. Sometimes the rehearsals are more for the benefit of the band than the new guy, gives them confidence you'll be able to hack it But it really takes away from learning tunes.
The trick is to play like a scalded cat, lay down a good enough groove/foundation, a few mistakes that are quickly recovered from are easily forgotten. If you're good at covering up, the audience will never know. Most of the time civilians are oblivious to all but the most glaring mistakes.
The other point, if they have a rotating cast of characters, they can't practice harmonies. They never know who's going to be on any particular gig. The only way that works is guys grab a part, the others see what parts are missing and add them on the fly.
This setup tells me they're mostly really good freelance guys, each of them playing with many bands. Leader goes down his list of the various components, bass, keys, guitar, drums, etc, from the top guys down, and whoever is available does the gig. These guys can play very well by ear, they listen to each other, they cover mistakes very well, they play with many bands, so have a huge repertoire.
You can start in the "C" or "B" list, and once you're familiar with the material, and have a few gigs under your belt, move up quickly.
My guess is these guys will understand a few mistakes if you play with energy, rebound from the mistakes quickly.
Do your best, have fun, play hard. The trick to this sort of situation is to not fret over the mistakes. While you're beating ourself up or getting upset over the last mistake, it takes your focus off what you're doing, leading to more mistakes. Once they're out there, you can't take them back, don't concern yourself with them, listen harder, keep going. Beat yourself up on the breaks if you must. Have fun, be fun.
Randy