And Protonmail offer 99.95% according to theirs (as of 2018, latest info I could find) — https://protonmail.com/blog/protonmail-reliability-sla/

99.95% is a high standard already: 365x0.0005x24 is 4.38 hours outage for a year. In our current world, 4.38 hours is certainly not enough time to, say, rebuild a DB from backup, if needed.

Yeah, especially when they're saying it's calculated on a calendar month, so you can't even "draw down" on previous months with 100% uptime if you have a serious outage.

Yes, but I assume the rationale here is that they will only have to refund that month and can reset the clock for the next month. So if they have serious downtime it won't cost them for the full year - just for that one month.

Yes, you fail over to a hot mirror, or if you really have to rebuild because something went nuts, do it from a recent snapshot.