Ah, so the extractor thread is toast, now I get it. Couldn't you just pull the spindle out of the bb anyway (with the ds arm stuck, that is, like a 2-piece crankset)?
As for loosening the stuck arm, if you really have to, use penetrating oil, as was already mentioned, and then give it some time, maybe warm it up with a heat gun and/or give it a cautious(!) spin around the block (or on a trainer ... or just rock lightly on the pedals in 3/9 o'clock position alternatingly).