You're on a fairly tight timeframe there, aren't you? If you're not going to do something like a stump-grinder, then that about leaves burning. One way to help that is to drill large deep holes downward all through the stumps, and then fill them with a nitrate. Saltpeter (sodium nitrate) is good, as is ammonium nitrate. You could get either of these from a garden supplier - they are fertilisers.
Now, what the nitrate does is to add oxidiser to the stump. If you can, if it's not freezing too much, pour hot water on top of the holes to dissolve the nitrate and carry it through the wood, then refill the holes with nitrate and repeat until the wood won't take any more.
Now let the stumps dry. If they're getting rained on regularly, cover them with some old plastic or something like that.
That oxidiser is getting towards the recipe for an explosive - gunpowder is saltpetre, charcoal and sulphur. ANFO is ammonium nitrate - fuel oil. Which reminds me - don't add diesel fuel or fuel oil to the ammonium nitrate in the stumps unless you're prepared to have them removed RAPIDLY. However, just soaking the oxidiser through the wood and then drying it out should just make sure the wood burns hot and steady.
Then pile kindling around the stumps and set them afire.
Remember you're trying to get the fire to chase the roots down into the ground and burn as much of them as possible too. DON'T be surprised if it happens - watch for it. Otherwise you could be surprised by a fire burning slowly undergound for months, then breaking out again. When you think the fire's all burnt out, dig after the roots with a mattock (do you call them that there?). Either way, you want to break as much as possible of leftover roots out, or douse fires burning undereground.
