Heating isn't really a problem. In fact a little gentle heating will help as the steel bolt and aluminium alloy carrier will expand at different rates and help release things. These bolts have thread lock applied on assembly and this is usually why the are sometimes hard to remove. Heat is a good way to release its grip. Even pouring boiling water over the offending area can do enough to get things moving.

Before going down the welding route, try knocking in a a Torx bit (that is slightly larger than the rounded recess) with a hammer and then use an impact driver to break it free.

Some people might suggest using a left-handed drill bit, but the bolts are very hard and if it doesn't bite and wind it out you're going to be left with a bigger problem because they'll be nothing substantial left to get hold of.

If all else fails, then tack on a bolt as you suggested which almost certainly will do the trick as the heat generated will probably be enough to free it off. Just don't go mad with the welder; a couple of blobs will do the job.

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