Thank you for your attention Bubblesoft :)
it seems I stumbled upon a solution or at least a workaround while researching the answers to your questions.
1. Yes, it is licensed. No "transcoding" message.
2. the CCwGTV starts the sceensaver/ambient mode after 10 minutes. I never realized this because my screen is always off when casting audio.
3. It always happens after exactly 10 minutes.
I activated "developer Tools" on CCwGTV. Now I was able to disable the "Ambient Mode". Lo and behold, the pausing 10 minutes after launching the BubbleUPNP-App on Chromecast has not happened again. Verified by switching back and forth.
On other occasions I had the impression, that casting to the CCwGTV is somehow different from running an installed app.
The state of the CCwGTV is not "playing" when casting to a "not installed" like the BubbleUPNP app.