Any question answered...

Also as a secondary question are things like JB Weld/similar strong enough to glue two nuts onto the thread of a bolt to create a new bolt head you can get a socket on?

You are much better locking the two nuts together by tightening them into each other and using high strength threadlock to assist. the majority of the lock force will be the two nuts being tightened up to each other.

Loctite 270

If you are drilling stainless, you'll be needing cobalt bits.
They're generally spendy.
Axminster Tools may be your best bet for individual bits - as opposed to sets.
Were it me, I'd go for the 5.5Ø and see if I can pick out the remains - as opposed to cutting into the aluminium.
https://www.axminstertools.com/axminster-cobalt-hss-drill-bit-5-5mm-400166?glCountry=GB&gclid=EAIaIQobChMIutrn65bI6wIVUed3Ch0X9wrkEAQYBCABEgLN2_D_BwE

Steer clear of the “***” (or something similar) branded screw removers they sell online and at Screwfix. Piece of rubbish drill bit was manufactured off centre by >1mm, so ofc it snapped the first time I used it. Screwfix dude said not to bother writing a complaint because their management don’t care.

Edit-dunno why everyone else’s replies just loaded...

There are a couple of ways you could do it. In no particular order:
1) Try a left-handed drill. I've never used one but they usually start drilling, grab a bit and wind the remains of the stud or bolt out. They're supposed to be pretty effective.
2) Drill down the middle and use an Easy Out. I've had mixed success with these with both cheap and branded ones. I'd recommend branded ones with the shallow flutes like Dormer. The key to these is a) drilling the right size hole and b) knowing when you're giving it too much welly and making a judgement if it's going to break. There are some tips to making these work successfully.
3) Drill it all the way out and helicoil. Easy peasy and always works.
4) Weld a nut on the bit sticking out - works really well but only if you have a welder.
5) Actually, if there's some sticking out you might get away with a stud extractor. Search for Irwin stud extractors.