I've been doing my homework:

1) Changing insert for insert-before-markers is problematic because it
somehow wreaks havoc with font locking. I guess some other markers are
being moved around besides he-string-end, maybe the one pointing to
the end of the prompt or something like that (since everything gets
colored the same the prompt is). Moreover, the he-string-beg marker
would still point to the wrong place no matter what its type were.

2) This will work for the end marker: (set-marker-insertion-type
he-string-end t), but it's too invasive regarding hippie-expand. Also,
it assumes the user is indeed using hippie-expand. But maybe changing
the insertion type of he-string-end just around the
delete-region/insert and just in case hippie-expand is currently
provided will do the dirty job. Again, this doesn't fix the
he-string-beg marker at all.

3) Same as (2) above, but saving/restoring the *positions*  of both
he-string-beg and he-string-end. This is the best solution I could
figure out till now. Take a look at the attached patch, please.

What do you think?