If you only need power and cadence then TCD is enough, but very expensive for what it offers. A bit expensive but a lot more features you get with Garmin Edge computers ...
Are you sure you need to remove the spider in order to remove the chainring? On Gen2 Levos you can get it off without removing spider although at first sight you can tell it wont come off, but with little fiddling it goes...
Yes on front, but on the back I dont see point for 220mm... 220mm rotor gives you 10% more breaking torque with same pull on the lever, bigger rotor also disapates heat better.
Another possibility will be a leaking o-ring on the plunger inside the lever, I also believed this was the problem on mine so I bought a complete new RSC lever, installed it and shockingly lever still goes to the bar ... and then I somehow figured it out that inside damaged brake hose is causing...
It should have already routed nylon cable guides, however I suggest you use your old cable to pull new one in using RockShox Barb Connector or similar ...
Find new M8 chainring bolts and screw it right into the new chainring. Many Levos come stock with chainrings with threaded holes, I guess due to low parts availability they also mix some stock parts within the same models ...
Frame has a lifetime warranty.
If running out of warranty worries you so much I suggest you sell it now with some warranty left and buy a new one with brand new warranty ...
Another factor to not consider carbon frame is ugly raw carbon finish on these bikes, every time I see someone asking if those carbon patches are normal, I'm glad I have alloy frame...