Features 1 and 2 should be quite easy to implement.
I’ll post something shortly to address these issues.
I do see your point about Feature request #3 and it took me a while to find it when I first started using Audacity.
However it is not (currently) possible because “Regular Interval Labels” is a Nyquist plug-in.
Nyquist plug-ins can be one of three types:
“Generate” type plug-in (will usually generate sound into an audio track)
“Process” type plug-in (usually an “effect” that processes existing audio data)
“Analyze” type plugin (usually outputs text, labels or files but not audio)
Audacity places Generate plug-ins in the Generate menu, Process plug-ins in the Effect menu and Analyze plug-ins in the Analyze menu.
Because this plug-in outputs a label track and not audio, it is classed as an “Analyze” plug-in.