1. depends on if you need to build custom modules/plugins. Alot of the functionality that you seem to need can be created using already created modules(plus my experience is that if I'm thinking I need to build a module one is already out there.)
2. I've heard they are trying to make this better, but it can be wonky. A lot of times developers will wait 3-4 months to allow modules to catch up to the new version.
Another option may be Backdrop (backdropcms.org). Which is working on maintaining backwards compatibility.
Another option may be Backdrop (backdropcms.org). Which is working on maintaining backwards compatibility.