Originally Posted by SilverChris

I book my straightforward personal trips directly through MH's website, but I also have a brick-and-mortar TA for other trips.

Purchasing channel has nothing to do with IRROPS. On the day of travel, MH will rebook you as necessary regardless of who issued your ticket. You might be thinking of schedule changes (in advance), which must be sorted out through the agent/airline that issued your ticket.

I'm sure you've been around FlyerTalk long enough to know OTAs are in general an absolute drag to deal with, not to mention their deliberate lack of transparency.