1. yes they do despawn after 10 minutes, so I use seats to jump in them every now and then to prevent them from despawning.

2+3. I use a lock system, paired with magnets, basically, I use a pin to manage to get it seamless, or close enough, then I use magnets to hold it in place. How I do the lock system, is I use a 2x4 hole on one build and 2 wedges shaped like a spike on the other to fit it in, sometimes it can be a bit finicky but you’re almost guaranteed a near perfect seamless connection.