Runako

Mate, where do I start. A lot of this you will absolutely figure out.

Rear brake - you can adjust the lever down depending on the type of rearsets you have (in general most are adjustable). However, you have just mentioned one of the things you will eventually learn. For better control and movement on your bike, you should get used to sitting on the balls of your feet. Its feels akward and sometimes difficult but you'll get use to it or adjust the pegs to suit. By doing this you will find it easier to manouver around the bike (especially when cornering at speed).

Engine braking - Try out another V Twin and then an I4 to get a feel for the difference. Better yet try another SV and compare to your own. The should feel the same as far as engine braking goes. If not then as some suggest it could be a problem with brakes.

Comms - You probably won't find something suitable with this budget. No point getting something cheap that doesn't work effectively or is uncomfortable. Why wouldn't you want to go into a physical shop?

Wind noise - I have a different (cheap) suggestion. Use a balaclava over your ears/earphones. This will reduce the windnoise (not eliminate). You can pick one up for as little as £2. Otherwise, as suggested above.

Fuel Eff - Depeding on how you ride, up to 46 MPG in my experience. And no, lots of engine braking doesn't mean less fuel efficient. In fact its not bad compared to some I4's. There is a chart somewhere online. Or do a search SV650 Fuel Efficiency.