Do jumping trains crash into each other?
Not yet, coding wise it's not a simple solution (at least not for a small brain like me 😬) since Factorio doesn't have a real vertical axis. The train jumps as they currently are are mostly a visual illusion.
How is train pathing with the ramps?
Trains still follow Factorio's innate pathfinding, meaning an automatic train will never find a path from one rail network to another if they aren't connected (eg on two different sides of a lake), even if the ramp would allow for movement between them. To make a train take a ramp, you have to guide it there using a train stop placed behind the ramp and adding it to the train's schedule
What items work with the Primer Bounce Plate?
Grenades, capsules, land mines, rockets/nukes, artillery shells, tank shells, and follower bots should all work, modded or vanilla, so long as they have a the right kind of action associated with them. For example, fish and artillery remotes, which are both technically capsules, do not do anything on the Primer pad because they don't have an action that makes sense. Follower robots spawn not locked onto any particular player and while they will still fight biters, they just slowly wander until they expire.
How's UPS performance?
It's okay. I personally never had any issues and most people report no lag until around megabase level. Unless you're spamming throwers and throwing hundreds of items per tick, you should be fine. I don't know a lot about optimizing code so I wasn't actively looking for ways to make everything UPS friendly. Though smarter people than me say I did a pretty good job (or at least not a horribly inefficient job) so I'll take it 😎.
Can you still be hit by trains while ziplining?
Yes. Visually to me, it doesn't look like the player is high enough off the ground to avoid being hit by trains so for now yes you can still be hit.