Let me see if I can take a crack at your questions.
First, GWS documents their motors extremely well. This link takes you to the page that shows the 350 motors:
1) Can you change the gearbox - yes, though it is probably just easier to buy a 350 with the gearbox you want.
The 350 with the 2.8:1 - They suggest a 6X3 prop
They tested it with a 9.6V battery which pulled 5.9 amps
So, if the 5.5 size prop is your priority and the 7.4V battery is the one you want to use, then you probably want to go direct drive. GWS calls that the 300 package. Let's look at the charts:
A 5X4.3 on 2 cell lipo will pull about 7.6 amps or about 54 watts.
Looking at the charts you can see the effect of changing the battey or prop.
Does this answer your questions?