Why So Much Rape in Stormblood

[Mahri]

I don't recall Arenvald saying his mother is dead. Did he bring it up? I thought he was merely abandoned by her, or he ran from her after getting the scar on his head.

Being 'fridged' effectively or not would require her to be dead or something of a similar tragedy. Her being alive but hating her son isn't fridging her in any sense of the trope.

It's inflicting something on an out-of-focus female character for the sake of tragedy that primarily affects an in-focus male character.

Yes, it is that trope.