First, back sweetening is easy with juice concentrates. Depending on what flavor u want u can add as little or as much as u want.
Two, my only experience was with Nottingham and it worked great. Either way for a one gallon batch, do NOT use the entire packet.
Three, pasteurize only if u want a carbonated cider. You'd pasteurize when it hits the level of carbonation u want. U CAN pasteurize now since it was fresh pressed to kill off any wild yeast and bacteria that may have been on the skins of the apples, but it might be easy and better to use Campden tablets and wait 24 hours.
Four, as far as how to pasteurize, do a search. I'd recommend searching for the "Stove top pasteurization method" and read most if not all of the thread. It will show I everything u need to know, especially how to prevent bottle bombs, which are a part of the bottle carbing. Or u could just make a still cider and avoid the carbing and risk of bottle bombs altogether.
I'd also read this in its entirety, it gave me a lot of good ideas and info.
And for some more good cider stuff check out this recipe and thread. A lot of good feedback from people about modifying the recipe.