1. O365 usually works fine for the online portion on most browsers, so that should be okay for their use. And won't require them to change habits in terms of how to use the software. (Bonus: cloud storage of their documents.) Only downside is they'll likely need an active internet connection to do anything
2. This one is tricky. I've had mixed experience getting newer Adobe products running in Wine, but it's been awhile for me so I'd say try it yourself. There are probably a bunch of good FOSS/cloud options available nowadays too if annotating, commenting, etc that maybe others can elaborate more on
3. Easy peasy, Brave does work well and should help them avoid malware on "those websites"

I'd say any LTS release you can get a working setup of Adobe in should be fine for them. 90% of what they're going to do is probably via a browser so it's OS-agnostic. I'm fond of Debian since it's very stable, but it comes with the drawback of older packages as time goes on, though you can pull in repos for more recent stuff for most important things.