Hi and welcome to the club. I'm going to be riding from Exeter to Peterborough later this year so good to know how long to plan for the journey
1) There are a couple of respected bike seat specialists that will refoam and recover your seat for you according to your requirements I've used
Tony Archer who was very reasonably priced and did a good job, others have had good results from
Digger 2) I wander if you won't get used to the change in riding position after a while as with any new bike it will take some adjusting to but will eventually gel. You could look at rearsets but I think they will have more of a horizontal affect than vertical, but I've no personal experience of them so can't say for sure. Gilles did make them for the Gen1 but I suspect you'd be hard pressed to find any now but you might get some Chinese ones from eBay.
3) Screens are very subjective, I am one of those that prefers a shorter screen, I have the streetfighter screen which basically fills the gap in the fairing and comes no higher. My good friend AndyW has just invested in a Puig DB screen after having tried a number of alternatives and is very happy with it, he is over 6".
4) Cold hands - I don't ride when it's that cold as I don't enjoy it and for me my bike is all about enjoyment.
5) I don't know if a change of bars / bar position may help with your joint aches as well as a change of screen?
6) I've just posted on adjusting the clutch in the other thread about the same issue.
Hope that's of some use.