As far as trying to take the tire off the rim, I just wouldn't. They are VERY difficult to do with a couple of prybars on a steel rim. On some of the alloys, because of the way the bead locks in, you're not doing it without the hydraulic bead breakers the tire shops use. However, cutting the sidewalls of even the best multi-plies is pretty easy. I would cut out a hole/strip big enough to pack in the wood or whatever flammable material you're using to light it up. I once cut out a couple of makeshift shoe soles in about 5 minutes just to see if I could. (cut out some soles, poke holes, add para cord and you've made sandals). A tip - angle the blade sideways when cutting the sidewall (or any heavy material for that matter), it lets the cut rubber offset so it's not just pinching the blade. Mucho easier!

As far as the alloy rims burning. I recall that in the earlier days of the new magnesium rims, there were some spectacular fires that couldn't be put out in some car races. "Magnesium" rims are not magnesium anymore because of that (and also because they would get micro fractures and cause the wheel to fail), they are aluminum alloys. Filing away to get some firestarting dust will just waste your time trying to light up aluminum dust.