Are all sidequests available until the end of the game or will some be locked off or automatically failed if I pass a storyline point?
No. Quests do not expire in that fashion. The most strict quests get is that some can only be done at night.
I thought there were some miss-able ones but they ended up working out in the end. Some missions just have some prerequisites before they can trigger.
All quests can be completed in any order. But I know that some quests if not correctly completed won't show up on your side quest completion list. I have two that I can't re-initiate or complete.