Another thing to try. Since aluminum expands more than steel when heated, the plug will "grow" more than the tube when heated. So, maybe if you can put the tube in a freezer, or chill it with liquid nitrogen, the plug should shrink at a greater rate than the tube, thus creating some clearance between the threads. The ideal situation would be heating the tube and chilling the plug at the same time. Sometimes, drilling a large hole out of a plug will let it collapse a little and turn loose. I dealt with stuck threads for about 35 years, doing industrial maintenance work, and very rarely did we have to drill all the way out to the threads. Repeated cycles of heating/cooling along with frequent application of penetrating oil nearly always worked for me, even it it took a couple of days for the oil to creep in. Heat, cool, bang on it, oil it again. Repeat until loose.
Since I've retired, I don't get to do this on a daily basis, but last night I encountered a light bulb that is stuck in a lamp next to my recliner. Headed to the garage to look for the oil and impact wrench.
Wish you were closer. I'd like to try my hand at it.
John