> Rather than increment CHARS_MODIFF separately,
 > it should copy the value from MODIFF.
 > That way, values of CHARS_MODIFF will be comparable with values
 > that were copied from MODIFF.

Yes, but it will deprive you of eventually introducing a variable
SAVE_CHARS_MODIFF and tell whether an insertion/deletion occurred
since the last save because CHARS_MODIFF > SAVE_CHARS_MODIFF.

 > Also, instead of setting CHARS_MODIFF explicitly in each place that
 > modifies the buffer, modify_region could set it from MODIFF, and
 > changing text properties could save and restore it to prevent it from
 > being incremented.  That means changes in fewer places, and no need
 > for each place that modifies the buffer to know about CHARS_MODIFF.

Perhaps the cleanest solution would be to equip modify_region with an
additional parameter, say preserve_chars_modiff, and do everything in
modify_region.

In the attached patch I tried to follow your suggestions.