Most Significant Carrier (MSC), what OP likely is referring to as "over water" is not relevant to this ticket as it originates in the US. The allowance for the entire ticket is the allowance for the marketing carrier of the first segment. Thus, AC allowance in J for IAH-IST. That allowance is 2 bags @ 70 pounds. The allowance applies for all segments.

Contrary to popular belief, carriers are free to require an add/collect for bags over the allowance enroute. But, they rarely do it.

Here, none of it matters because the allowance was 2 bags @ 70 pounds.

Don't bother complaining to AI. Complaining about incompetence to a carrier which has raised incompetence to an art form is not worth your time.

Do file a complaint with US DOT. Can be done online. It needs to be about 1/10th the length of the OP and focus on the violation, not who said what.

DOT will require an explanation from AI and sooner or later will impose a fine if there are enough complaints.