Yeah, so short version: after 4.2, the old getContent hack just stopped working because WordPress rewrote how MCE views work. You can’t just override the template anymore without re-registering the whole gallery view. There’s no clean hook, no magical filter, no partial override, you either unregister and rebuild it like you did, or you live with the default. Everything else like extending or patching after init is a no-go. It's messy, but that’s the only way it actually works now.