Sorry to hear about IMF. - have you been able to withdraw and obtain a refund on medical grounds?
Five years ago My daughter had to have an operation to remove infected bone from her ear following an earlier failed op to clear Glue ear and sticky ear drum.
After the op she was fitted with a silicone ear plug, the hospital took a cast of her ear and the plug was made within 10 days and was completely water tight, she does not need it now. May be worth enquiring at the auidometry department of the hospital.
On the general swimming points it may also be worth choosing your IM swim venue, obviously a sea swim can have serious tide and current issues, there is a bit of added benefit from the extra buoyance of the salt water BUT this could be offset if there is a strong current.
It also helps if the course is the correct distance unlike IMUK last year.
As suggested above you could try breast stroke,
If you join the gym I go to there are plenty of women in there who swim and never get their hair wet so cant possibly get water in their ears, I could always ask them to give you lessons. 