disassembly
there are 7 screws:
1 torx screw (t5):

2 under the g900 label, 1 under each side feet, 2 under the sides of the bottom feet. you do not need to remove the top feet.

the shell falls apart pretty easily after unscrewing these. there are two cables at the base: one ribbon cable for all the switches/lighting of the top shell, and one power cable for the battery. the ribbon cable is trivial to unplug: just lift up the flap. the power cable connector is very tight. be careful to not break the pcb when unplugging it; the pcb is very very thin.

top piece:

battery is attached via some double-sided tape. you can pry it out with a bit of effort.
if you plan on reattaching and removing the battery in the future, i recommend peeling off some of the double-sided tape

once you unscrew the ten screws near the edges of the top piece, and the single screw under the hole covered by the battery, the top shell should be loose. wiggle it around a bit to figure out how to take it off.

oh hey... look what we've got here

someone was careless in the assembly line and got a spring onto the magnet for one of the side buttons
nearly full disassembly:

standard 20m omrons:

button hinge:

the bar takes a bit of force to push out:

bottom piece:

full disassembly:

wheel pcb top:

wheel pcb bottom:

main pcb top:

main pcb bottom:

weight
no pic, but with all the feet and screws in place, and the standard configuration of side buttons, the g900 weights 107.0g on my scale.
with top, bottom, and side feet; 7 screws; and side buttons/covers removed, 103.2g remain.

top shell is 64.8g

battery accounts for 14.4g of that

bottom shell is 38.4g

of which 9.9g are due to the scroll wheel

there are 7 screws:
1 torx screw (t5):
2 under the g900 label, 1 under each side feet, 2 under the sides of the bottom feet. you do not need to remove the top feet.
the shell falls apart pretty easily after unscrewing these. there are two cables at the base: one ribbon cable for all the switches/lighting of the top shell, and one power cable for the battery. the ribbon cable is trivial to unplug: just lift up the flap. the power cable connector is very tight. be careful to not break the pcb when unplugging it; the pcb is very very thin.
top piece:
battery is attached via some double-sided tape. you can pry it out with a bit of effort.
if you plan on reattaching and removing the battery in the future, i recommend peeling off some of the double-sided tape
once you unscrew the ten screws near the edges of the top piece, and the single screw under the hole covered by the battery, the top shell should be loose. wiggle it around a bit to figure out how to take it off.
oh hey... look what we've got here
someone was careless in the assembly line and got a spring onto the magnet for one of the side buttons

nearly full disassembly:
standard 20m omrons:
button hinge:
the bar takes a bit of force to push out:
bottom piece:
full disassembly:
wheel pcb top:
wheel pcb bottom:
main pcb top:
main pcb bottom:
weight
no pic, but with all the feet and screws in place, and the standard configuration of side buttons, the g900 weights 107.0g on my scale.
with top, bottom, and side feet; 7 screws; and side buttons/covers removed, 103.2g remain.
top shell is 64.8g
battery accounts for 14.4g of that
bottom shell is 38.4g
of which 9.9g are due to the scroll wheel