Regarding your questions about the TKC's,Noisy,I am quite happy with their road performance-ride and adhesion are very good-no classic knobby 'lumpy' feel or borderline traction concerns.....This is surely attributed to the low height of,and close spacing of the cleats. The TKC's are described as 50/50 (on/off road) tires,and,IMO,they are(much) better than that as far as road manners are concerned. But,also contributing to the situation would be the RE's weight and low power output. If the Scrambler had a much higher power to weight ratio,the TKC's would very likely be 'borderline'.....
TKC tire noise?-Zero to minimal-I don't know,as my exhaust note overwhelms any of that. Because of the cleat design and placement,I'm sure that it's minimal....
K761-My impression is that these tires are either 60/40 or 70/30 -a far better road tire than an off road tire.
One question to ask yourself in order to better decide which tire, is-"How much paved road use versus un-paved surface use do I expect to encounter?"
As you already know,there are no significant(meaningful) tire width options because of the stock swingarm's limitations. There is a relatively simple mod to improve that situation,but the stock S-arm is still the show stopper in using a significantly wider tire.
Suspension- Fork springs-I have used Progressive units many times. I'm not too surprised about their lack of involvement with RE's,but,you may still be able to make use of their products but discussing your needs with their Tech Dept. They''l likely need info such as RE spring length,I.D.,O.D.,Bike & rider weights to calculate an alternative spring from their existing inventory. The use of pre-load spacers is 'low-tech',but often is a quick improvement. Obviously,too much spacer length can lead to spring coil bind under full fork compression,so watch for that possible problem. Generally,pre-load spacers are expected to offset suspension sag resulting from age or sheer machine weight at rest. Typically,the spacers will accomplish that without creating said coil bind.
Rear shocks- 1/2" to 1' longer perhaps,in the attempt to raise the rear to match the height change made in the front. Additional rear height will reduce your fork rake,and quicken the RE's steering..maybe an improvement for you. Consider buying shocks that are rebuild-able and that offer internals for that purpose,so that you may carry such on a long trek....
Hope that the above points are helpful.....