Uncommon bolt head

It's called a spanner slotted (or slotted spanner) head. Useful site here. Hard to find in the UK, why the owner used one I cannot imagine. Screwfix doesn't have them. This site may do, although it is international. You'll need to figure out the correct size before you order.

Alternatively get a real cheap flat-head screwdriver the right width and file down the middle until you get the right notch.

If you can grip it with a pliers try a pair of locking pliers and unscrewing it that way I keep a mini vise-grips handy for that sort of thing.

First. I would ensure that you are using a 4-way Tire Iron. It makes you apply equal pressure to the bolt and you will be amazed at the different. In fact, if you have not tried this. Then i would not worry about the heat source for now.

In terms of the blow torch. you need anything that can throw enough heat to make the bolt almost red hot (note I say almost)

A butane torch will be totally fine. Just watch out, the hot nut will be a little easier to snap when hot.