Yes for sure you can use a small propane torch or even a fine brazing tip on your cutting torch to heat. I used to use one of those self igniting tips on those miniature propane bottles, but once I got the first butane one which are so nice for smaller work, I no longer keep any of those little bottles of propane around anymore. Just several cans of butane instead. BTW, with those butane torches, you get what you pay for, not sure what the make is, but the best ones are the black ones with the metal body. They are a lot more than the cheap crap but will last you for years...as long as you don't drop it too often on concrete or steel:22:
Here is a pic I found online of the good one. I'll try to remember to check in the shop later, I have a brand new spare one, I'll check the name on the box, as I think the stickers long ago fell off the one I use.
As for holding the lug on the cable, I always stake them first with a suitable sized pin punch, at least once in the middle of the barrel. Larger ones will do twice. If you use too small of punch, you will end up puncturing the lug as well, which then makes soldering a *****. That way you have both, a friction pressed fit and a completely sealed soldered connection as well. There is a special tool available specifically for staking or crimping a battery cable end, look something like a cable cutter used to cut steel cable. I've considered getting one, as sometimes when staking with the punch as I do, I could use a third hand lol.
To hold the cable and lug in position for soldering, I usually just clamp the cable lightly in the vise and bend to position as required. Or use a vice grip to hold it in an appropriate position if still on unit.