"Thorns" ...Just wanted to second the Schwalbe Marathon recommendation. I've used them on a LOT of bikes, for a lot of miles, all over the US (our folding brompton bikes have them too) and it's incredible what they can withstand. Have also had sealant inside a tube before, not sure if it ever did any good, because I also had Marathons (the MTB version, which is an interesting tire for another day) on that bike. I don't see tubeless being your answer there. Big cities, in my experience, = flats. There's always glass and metal around. Boston was no different when I lived and commuted there for ~9 years. Tons of flats until Marathons, then zero.
"Dual front chain rings" I'd be a little surprised if the same chain length worked for big rings within your dropout length, but I haven't done the math. If so, then cool! And if you liked it, I wonder if (and here I get out of my mechanical depth) you could use a front derailleur and a chain tensioner to be able to switch on the fly?