Heaters are 1 of the weakest pieces of equipment in our hobby & have been for many years! Your fish will likely be fine, or even better than at the high temps. Don't stress out over this as far as your ich tx goes...High temps only knock off a couple days off what you need to do...you know, WCs & vacuuming & keeping the salt dosage up! Don't despair! You are getting closer to an end to this ordeal but you have several days to a week or so to go. Keep it up!
98% gone means nothing yet, it's just what shows on the fish's exterior parts. You're far from done...lots of work ahead, but you can do it!