About modinfo - I think I'm the developer who most recently made significant changes in that area of code a few versions back (modinfolib), and I say, feel free to just set it to null.
I did so today in fact for a stack of courses on our live system regarding problems with other data that I had to fix with sql queries.
However:
1) If you're doing this from within code, you should use the rebuild_course_cache($courseid, true) function rather than doing the raw SQL to null the field.
2) If you're doing this manually and only need it for a single course, you do not need to hack the database. Just edit one activity on the course - any activity - and, without making any changes to its form, do 'Save and return to course' - this will regenerate modinfo.
--sam