Since the temp is not good for the eel, I think you will have to try meds. Look for some that are plant safe. If you are concerned, you can use half strength.
Just because you cannot see ich spots does NOT mean the ich is gone. It has multiple lifestages. You need to continue treatment for at least a week after the spots disappear if you don't want it to come back.
Also, if you can't raise the temp to 86 or higher, then IMO don't even bother raising it. The high temp kills the ich, but lower temps don't. So, I just suggest lowering it back down to the normal temp for your tank.
If you think the eel can pull through, then you could continue your present treatment, but it doesn't sound like the eel is doing well. Up to you. Good luck.