Lots of great stuff here. The only thing I would add is that if you don’t do band practice frequently (and you don’t – once a week is not frequent), then there are things you can do to help make band practice more productive.
Not judging, my band only practices once a week, as well. Less lately, because jobs n shit.
What I do, though, is I have a playlist of the setlist for the next show. That’s what we’re working on in band practice, right? Playing through the setlist for the next gig.
So I play through the playlist for the next practice. The night before is fine. I have practice tonight (hopefully). Last night, after my warmup, I played through the setlist, or at least as much of it as I felt I needed to (we have 18 tunes on the set right now). I focus on the ones with interesting or different arrangements, progressions, or chords, to get everything fresh under my fingers before practice.
At practice, if we fuck up, we back up. Some mistakes are tiny and you just plow forward. Anything that causes a ruckus in the group (more than one person’s mistake), we start the song over. Several times, if we need to, to get it right and tight.
That refresher can help with remembering arrangements and staying on track. It’s better to know the number of repetitions of something than it is to count on a certain embellishment (unless you work out beforehand that the embellishment is a cue). You can write out a map.
Intro (2x)
Verse (4x)
Bridge (1x)
Chorus (2x)
Intro (1x)
Verse 2 (6x)
etc. Follow the map.
For your singer, if she’s in pitch during warmup but not in practice, there may be an audible difference she’s just not recognizing – maybe she can hear herself better at home than at practice? Maybe the introduction of earplugs at practice is messing with her? Study the differences.
She should work to shorten her lyric “cheat sheet.” Start with a full sheet. As she gets comfy, only the first few words of each line. Then the first two. Then only the first word of each line. Finally, the first word of each section (verse, chorus) etc. as a prompt.
Hope some of these tips are helpful! Good luck!