spinning is good. but...

lower cog = less spinning. 42/15 is higher than 42/17. if you want a bigger ratio, get a smaller cog.

if you change the cogs bigger/smaller you can just slide the wheel farther back/forward in the dropout,yes.

i would say stay at 42/16, its a great ratio; try to aim for 65-75gi though, i wouldnt go higher. it depends on how hilly your area is. from what youre saying you dont have many hills. try out a bunch, cogs are cheap.