Excellent work!
I'm glad you think so!
I've attached a new file that addresses some of the issues you identify above:
--) It uses axis-group::width to box accidentals
Beautiful! Now there's really no need for the extra padding properties. (I've kept them in the attached file, but combined them into a single property which takes a pair = 'extra-padding.)
--) It creates a FrameStub grob to occupy the horizontal space that a frame line takes up (note that I don't know if this actually works - I'm not sure how to test it - but at least it doesn't crash!). Whenever you have a spanner that somehow occupies horizontal space at its bounds, you can create stub grobs that approximate this space. This technique also works for items that, for whatever reason, have heights that are difficult to approximate (see SpanBarStub and StemStub, for example).
This works great for spacing on the left side. It wasn't working on the right, so I added 'extender-length in (plus a slight correction accounting for arrow-length, etc., which should be fairly easy to calculate). Now it will push everything forward nicely.
One thing I've noticed, though, is that the line will not cross a bar line. If I increase the length of the extender, the bar line is simply pushed to the right. I figured out one way around this--namely, setting the X-extent of the bar line to (+inf.0 . -inf.0), which is what I do in the attached file. Of course this has other consequences. Is there any other way?
--) It gets rid of some dead or unused code.
Thank you very much for your improvements and explanations--I think I've learned quite a bit!
Best,
David
frameEngraver3.ly