The first suggestion cannot work with Shotcut’s architecture. Each clip object can only have one audio track in Properties. The only way it can mix audio is when there are parallel clips on multiple tracks. What could possibly be helpful is to somehow expand upon Detach Audio to let you select which of multiple (checkboxes) to detach at once. Plus an option to group all of detached clips with the original.
You can already attach filters to timeline tracks and the entire mixed timeline (Output in the top, left corner of the timeline). In fact, some filters can only be used on the track. Click a track header to select it and go to the Filters panel.
The next version automates this for gain by adding a volume slider to the track header.
And the Adjustment Clip feature in that next version is another way to add audio filters across all of the tracks–an alternative to Output filters with a different control over the time range.