Good find, that's not intended. I'll try to find a fix for it today.
Edit: Actually I think it is intended, at least in-game, to make sure that whatever is "behind" the surface elements gets cleared. I'm not sure if a giant screenshot without the blackness looks good, but I'll try it out.
Edit 2: https://github.com/OpenRCT2/OpenRCT2/pull/9422
1) It is supposed to only show rides that the scenario intended. However, I believe there is currently a bug in that, which causes it to show more.
2) This is something you should really have checked yourself. I will answer it this time: OpenRCT2 groups rides in much the same way that RCT1 did, unlike RCT2.
So I just finished linking up RCT1 to OpenRCT2, but I'm wondering, if I play the RCT1 scenarios, will they feature only the rides that the scenario originally had access to, or will it expand it? For example, if I start Forest Frontiers, will I be able to access rides like Enterprise or Jet Skis that originally weren't available for that level?
Additionally on a different note, will OpenRCT2 compile rides of similar natures? I like creating at least one of every ride, but when "Rowing Boats, Canoes, and Bumper Boats" are all seperate rides it not only clogs up research by unlocking rides that just use a different carriage design, but also just crowds up the building menu.