Re: 9.8HP Merc oil change
The "flush" screw is for flushing water thru the powerhead with the motor not running. Unless used regularly, usually will be stuck and difficult to remove. So don't mess with that one. It's better to flush the entire motor in a barrel anyway.
Your drain and vent plugs for gearcase oil are way down there, on the gearcase itself. In fact, in the 2nd picture you can see the vent screw/plug, just above your hydrofoil unit. The drain is farther down on the rounded part (aka "bullet") of the gearcase.
Here's a recent thread that you could consider "Gearcase 101". It'll run you thru the very basics of doing this simple but Very Important task:
http://forums.iboats.com/showthread.php?t=259698
HTH & enjoy your new rig!...........ed