OK..

I take it from your strategies used, that the hex bolt is countersunk? This is why some vicegrips weren't used?

I've removed frozen and broken high tensile bolts out of my engine block on a couple of occasions. Either your tools or technique are faulty

The tightness is either from threadlock, and/or aluminium corrosion.

Maybe you need to drill it out close to the diameter, but this is very risky, as the drill will be ecstatic to make it's way into the softer metal. If you drill a larger diameter, you can centrepunch and drill through to the alloy at 45degrees inside the large hole with a 3mm drill, do this once on either side of the large hole's diameter. You could then squirt some good WD40 in there? If you haven't completely drilled out the hex bolt, then the WD40 might sit in there. Leave it overnight and give the screw extractors another go. Depending on the diameter you might have to use the centrepunch to "shock start" the bolt to move.

Otherwise try zapping the bolt with some oxy/acetylene torch, being careful not to melt the Alloy bit.

Getting good quality extractors just has to work.