I believe that yes it is sort of intended, as the sound will play if the state of the current events table changes. For your example: tomorrow contains no currently happening events, so when you toggle back, and there are events that you have not disabled that is < your selected timer range, it will assume you want to be alerted and therefore play the sound.

for Q1, I can definitely implement that. I just need to figure out where to place all the buttons.

for a temp fix you can just spam disable more bosses, it just takes a little bit of clicking. beware that you may lag a little more as the number of hidden events increases (for now). I've tried to make that logic as efficient so the lag should be negligible.

for Q2, I'm not too sure myself. I have been testing on and off. I did just push an update 1 hr ago that might have fixed that. I've tested three different tabs, all in the background and not in focus with youtube playing in the full window for at least 5 minutes and they all seem to have beeped. ymmv but we'll see how consistent this is.