Ugh. When you say it is your first year, does that mean someone else in your firm has been doing it and now it has just passed to you, or it came over from an entirely different TPA? If the latter, then I'd be inclined to make it the client's decision - we'll fix for 2019 and 2020 - for all prior years, they should be fixed, and I'll do it (for a princely fee), but if you choose not to do it, you are liable for the consequences.
Alternatively, if they refuse to fix, give them their walking papers.
How big a plan, and how much interest might this turn out to be - lots, or "not too bad 'cause it is only 11 participants" or something like that?
These types of situations make me question my career choices. I should have been an optometrist!