Cold mix will work but as you said just take longer, warm the job as best you can, just let it work and see if you can find a tap extractor or make something to go down the flutes and try to screw it out, just give the Alum time to work and get on with something else, always perfect your technique before working on your project, i would be doing this by hand with the use of a jig to make sure everything is square.
Good luck
Regards
Mike