Best Practices for Multiple Part Scores

[address@hidden]

I'm sure this has been discussed before, but I've been having a heck
of a time finding previous threads that go into any depth.  I see one
from 2008 (revived in 2010) that references a part of the Learning
Manual that seems to have moved from section 5 but doesn't really
contain all that much information.

I would like to set a large piece with multiple parts, with the option
of producing single-part scores (e.g. an orchestral piece).  I have
found that very quickly my source files get out of control.  What are
the best practices I should be following to keep everything clean and
D.R.Y.?

I am specifically looking for advice on the following subjects:
- Time, key, and tempo changes (and any other "global" markings)
should clearly be specified in just one place.  Where is that, and how
should it be incorporated to the music?  I've been trying to copy the
"spacer rest" model, which seems to be the right idea, but that
disrupts any multi-measure rests on the various parts.  Another
possible strategy would give them to one "master" part and quote them
elsewhere, perhaps?
- Multiple parts per staff (e.g. four-part choral music on two
staves): how to handle duplicate dynamics, phrasing, doubled rests and
notes, etc?
- Short lived divisi - is there a better method than using < > on
every note?  What about a passage with rhythmic differences?  Must
this be separated into yet another part?
- File structure: one per part (makes sense to me) or one for the
score and parts pulled out with tags (suggested by the manual)?  Other
options?  How should I pull this all together?
- Multiple sections of a piece, each with its own title,
instrumentation, etc. probably should be separated into multiple
files?  What about taking into account above discussion of multiple
files per part?  Do we need a directory structure to handle all of
this?  How do I integrate them all to set one right after the next?
- Phrases or passages played multiple times by the same or different
parts: I'd like to abstract them out, but how do I incorporate them in
the proper point in the part and octave, etc.?

What is amazing to me is that LIlyPond is capable of just about
anything.  What is frustrating to me is that doing something that
seems to be standard in music notation (such as writing multiple
parts) takes so much manual effort and so many hacks to look good.
And there is much more documentation for strange things (like printing
notes with colored heads, weird times and misaligned staves, etc.)
than there is for the things you would actually expect a person to do.
 Perhaps this is due to my unfamiliarity more than anything.

Thanks in advance for your thoughts!
Andrew