I resolved this by logging into C Panel and restoring a back-up from a few days ago (Oh well…). I then deleted the Events Tickets Plus install (which failed and then I did another restore). Finally I rolled back to an stable enough version and it stuck.
It sucked that is happened, but I was able to repeat the original steps I took and it stuck this time. The only thing I did differently was to update Event Tickets (free) before installing pro.
I’d still be interested to hear from Modern Tribe why this might have happened in case this happens to another user in the future.