I've only had it happen a couple times, but as far as I remember...
A1: No. Only to the last waypoint loaded.
A2: No. Nothing will "automatically appear." You can delete the first part of the route in Route 2 and fill in the end manually, if you like.
A3: The SID or STAR will not be accepted. You have to manually delete sufficient waypoints to add any more in any manner.
I simply deleted waypoints at the end of the route, input the SID, and then input the remainder of the route some time after TOC.