<<<#3 If there is audio that is almost silent;
like -49dB, It would be nice if it could select all audio below
some dB limit and it will be set to true silence (-INFdB?) and all audio
that surrounds the “silenced” spot will be smoothly Faded Out/In.>>>
That one you can do with Steve’s Noise Gate.
https://forum.audacityteam.org/t/noise-gate/14017/1
Koz