Thank you very much for the patch!
The MySQL version is 5.5.61-cll. Host support did not report problems as per 2) and 3). I don't think that we can do dev server testing, because in order to have realistic results to compare, we'll have to duplicate our site on the Internet which does not sound like a good idea to me.
We could eventually switch our site to use an InnoDB copy of the database for a while to see what happens. Wonder would it be wise to do it?