IMO it looks like a 2.5" might not clear the seatstays, or will at least be too close for comfort. Won't know unless you try, I guess.
In my experience if you're going to have a second cog on there you should have an FD. The idea of having a bailout chainring up front without the need for a shifter and FD sounds appealing up until the point where you actually need to use it. Stopping, getting off the bike, derailling the chain with your hand, are not anything you would want to do on tour. If it we're me I'd just get a suitably large Sunrace 10 speed cassette (11-42) and a narrow-wide single chainring, either 36t or 38t. With MTB tires you really won't need any gear combo above a 36-11.
I just installed a slightly older Suntour XC Pro headset with the Grease Guard licensing from WTB and it works really well. The fact that the design stopped being popular is more due to the ascension of cartridge bearing headsets than any flaw in the Grease Guard design. It will last just as long as a King headset if cleaned and serviced periodically. Do you have the bearing cover for the top? Don't see it in your picture.
Last edited by TenGrainBread; 04-25-19 at 06:41 AM.