Does the higher stacked RAM add any (extra) fragility to the player?

The stacking itself shouldn't - you've got 49 soldered connections holding each layer down. What will add fragility is that each layer takes the RAS wires closer to the IDE cable and that vibration could be transmitted through the cable to these wires that could cause them to work loose or (worse) pull a RAS pin off of a chip. This risk will be mitigated by gluing the wires to the tops of the chips where they cross to prevent them moving. I'm also going to run a length of electrical tape across the top for extra protection. Hmm. Maybe I'll hotglue a small piece of polycarbonate across instead - less 'gooey'.