When multi-cam editing (like - YouTube): Q1: Is it possible to set the timeline to play nonstop without being affected by double-clicking the mixer viewer? Q2: Is there any shortcut (hotkey, eg: press 1 2 3…) equivalent to double-clicking the mixer viewer? Thanks 
1 - can you set the timeline to play nonstop - I do not think so, but there may be some trick that I do not know about.
2 - shortcut instead of double-clicking - I do not think so, and I am not sure what could possibly be shorter then a simple double click?
Perhaps someone else has discovered something I have not thought of or forgot
I dont think it can be done either. CinGGs multicam is set up rigidly and is not easy to change. There is a discussion about it on the mailing list:
https://lists.cinelerra-gg.org/pipermail/cin/2022-January/004556.html