Just some of my opinion regarding GMs:
We've seen Sherria, Lyon, other figures from Lamia Scale, but Ooba didn't appear anywhere this arc.
We haven't seen Bob from Blue Pegasus.
That's just an example, but i mean, during all the story so far, guild masters never played a big role. The exeption would be Makarov, since this is FT, but all side guilds' guildmasters never appear or fight, or help at all.
Despite the "war" aspect of this arc, this isn't any different. We have Makarov who had his moment, that's about it for guild masters. I consider Sting aas still a regular guild member, he was originally a member, a fighter. Now that he became GM he still is seen fighting, contrary to the stereotype of the oother GM that talk and slack off on back lines.
I definitely don't give any credit at all to GMs, the exception being Makarov as i said. The other ones did close to nothing during the entire series, and that didn't chnge this arc. They are even less important than regular mages imo.
I see no reason for Mermaid Heel's GM to randomly appear now, unless she's linked to a main character. Which i wouldn't really like, at all.
I just want Hiro to have forgotten about her to be honest.