I realize the lens you see this stuff through, based on past posts. All business, all the time, music as a full time job. Punch in, punch out, get paid and go to the next. I get it. But not every band is the same setup of 'rotating cast' working band functioning as a job. If it is, that's awesome. Go get it! I wouldn't want to deal with a flaky coworker at my day gig calling out all the time on a whim, or surprising people with dates off that they didn't know about far in advance... emergencies aside. Subbing someone in at my particular day gig isn't a thing, and in the bands I've been part of in the past - mostly originals focused deals - subbing someone in for those was more difficult as well. There was more to prep and communicate if it did become necessary.
I'm just sharing 'war stories' from a band I was in years ago where one member started doing stuff that we hadn't all collectively agreed upon, then got mad when we pushed back. And it's a fact that not every band is a 'working band' on the same level as other bands. It's crucial that collective expectations and goals be decided upon before giving this much control to one person. The band in question was one that wasn't easily subbed in on either, as it relied heavily on a book of originals, some often complex. It was RARE when we'd have to sub in someone, so it was a little different dynamic.
The approach we used back then, even with the Google calendar in play, was to say to a venue owner 'pencil us in as a hold, and I will confirm with you within 24 hours'. We lost maybe 1 gig as a result of that approach. If a venue isn't able to give us 24 hours to verify availability, then that makes me question their professionalism as well.
We worked PLENTY, with few to no issues, until the goals and criteria started getting muddied. Transparency AND clear communication are always key, as are the aforementioned discussions about the collective goals of the band.