There are many screws that hold the battery tray on. Take a look at this:
http://www.losi.com/ProdInfo/Files/8IGHTEexplode2.pdf
As for the diff gasket, I didn't have a new one so I let it ride, so far it has not leaked but I know from the CRT that leaks are not uncommon. A new one should tighten it up.
As for the motor bracket, I'm using it. Fits perfectly with Mike's mount AND the little riser under the diff AND my 2200KV Neu. It isn't quite touching but I figure if the chassis tries to flex too much it will hit and give some additional rigidity. Not a problem yet even over those big Wolcott Jumps.