DeltaMikeCharlie 1) Streaming results seem to depend on the client and/or the protocol in use.
I can confirm your finding #1. Today, I used two Kodi clients (Win+CoreELEC) via RTSP and one VLC client via HTTP to access the same ORF2St HD stream. All three clients handled the switch from national to regional with the usual glitches, but surprisingly, VLC even "survived" the switch back from regional to national, while both Kodi clients went black.
Here’s where it gets interesting:

It’s clearly not the stream itself causing the issue, but rather the subscription of each client. While VLC keeps playing on the same stream shared by the Kodi clients, the latter go dark. The screenshot shows that the Kodi subscriptions still receive the correct input bandwidth but have zero output bandwidth (see red boxes), whereas the VLC subscription maintains proper output bandwidth.
Maybe the problem isn’t tied to a specific client but to the protocol being used. Is there a way to get Kodi to play a stream over HTTP?
Additionally, I can confirm that the PID list only grows by adding new PIDs and isn’t cleaned up when PIDs are no longer in use. Since VLC continues to play using the same PID list without problems, I doubt this is a critical issue.