The PCB is held to the plate with your switches, which is top-mounted to the case.
Yep, I see that.. Curious if there is any flex since most people use standoffs on their aluminum case to support the center of the pcb, maybe it doesn't matter. Either way it looks like a great board. =)
Flex of top-mount is more or less the same or smaller, but better in terms of typing experience. The tray mount is more flexy on the edges (which doesn't mean it's better, it's exactly the opposite), while the top mount is more flexy in the middle (well theorically, but if it is brass it will be rigid anyway).
I believe standoffs/tray mount was something of the past and current budget cases and there aren't many people using it, it allows to make a case with a single piece while other mounts needs at least 2 pieces, but it will just end up sounding bad unfortunately