Howard Miller

Did you try restoring from the backup they gave you? You may well have been absolutely fine.

I've never needed to do it, but I can't think of any reason why you couldn't update the lastaccess field in the mdl_user table. As long as that's *really* all you need. There are dates for things all over the place.

It doesn't really help you but my advice is always NOT to modify core Moodle code. This is why.

I would seriously consider transferring your additional functionality over to custom plugins and, possibly, a custom theme. It's pain at the time but worth it. There's very little you can't do using plugins.

Nothing - unless MySQL 5.7 (which I assume the OP is running) gets some horrible security vulnerability in the interim. Because it isn't getting fixed.

I have no idea - I don't use Moodle Calendar or Google Calendar.

However, I did a quick search and there are lots of people (not just Moodle users) complaining that Google calendar sync is slow (or it takes many hours for new events to appear in the sync) so it may be a "thing". Worth doing a search yourself, if you have not done so already.

coursecreationguide is a site-wide setting found in Site administration > Appearance > Moodle Docs

You should simply be able to go to that settings page and make sure it is set to the default.

It's odd that it's not set, however.