Good question!

My personal feeling is no. Because you can still combine aspects from the base classes. However, if you want access to ALL combos withing prismatic, then yes. I infer this from where Bungie state about when you unlock an aspect and fragment, it's unlocked for all classes. Now this could mean fragments for all classes (hunter, titan, warlock), but then why include aspects in that sentence when they've always been exclusive to each class? My guess is, once we, for example, unlock Bleak Watcher on Warlock, we can use it within its stasis context, or in its prismatic context. And if you can't unlock it because you don't own beyond light, then you can't use that for buildcrafting your prismatic subclass. That's just my theory. Tl;Dr: you don't pay for it all, you limit your buildcrafting to prismatic to just light subclasses. In which case, I definitely see Bungie selling some sort of unlock option for owners of Beyond Light and Lightfall to skip and unlock both subclasses.

I would think no because those particular new subclasses are exclusive to final shape. Just like if you want access to Statis you have to purchase beyond light vice versa lightfall needed for strand access...