Stuck Compression Plug and Top Cap Screw Issue
My top cap screw bolt is currently stuck in my compression plug, and I'm seeking some advice on how to remove both the broken bolt and the compression plug.

This was the top cap I was using:
It's a weightweenie one piece top cap and top cap bolt.
https://www.cycleservicenordic.com/en/products/headset-top-cap/absoluteblack-top-cap-and-bolt-in-one-

What happened
My initial installation of the top cap wasn't perfect for the preload.
Symptoms appeared in the form of loose headset and the top cap bolt becomes stuck. 
Tried to unscrew top cap but didn't work. 
Eventually the T25 top cap bolt stripped. 
Used Araldite in the bolt and on my T25 key, left it over 2 nights, still didn't unscrew. 
Hence my diagnosis was that during installation, there was cross-threading. But does it explain why it is so insanely difficult to unscrew? Not sure.
Then came the drilling.
My plan was to drill off the top cap, then use some pliers to turn the remaining top cap bolt.
Look where it got me, there doesn't seem to be any top cap bolt left.
Specifics
It's a Bianchi Oltre XR4 fork. 
1-1/8inch steerer tube. Rim brakes (no hole in steerer tube).
Compression plug was torqued to 6Nm. Top cap was just lightly hand tight.
Compression plug is from Aliexpress and looks like :

Parameters
I am not planning to reuse the compression plug so it can be destroyed. 
Steerer tube can be cut about another 4-5mm.
Seeking your wisdom
I'm not sure if there was actually cross-threading or that the metallic surface somehow fused together (not entirely sure if compression plug is steel or alu). 
Please teach me how to remove the whole assembly out of my steerer tube.
Thank you, the Feynmans of the bicycle world!
Thanks for all your help!

What worked
Drilling with a small bit into the edges of the broken top cap screw. 
Miraculously, I noticed that the screw loosened itself as it's unwinding little by little from the vibration of the drilling. 
Then I used some pliers to unwind it out and was able to find a near-perfect hexagon hole where I can stick my 6mm in. 
Mis-diagnosis?
So...the top cap screw thread doesn't seem to be burred or damaged. Even cosmetically it looks fine. 
The black anodising on the thread is consistent and no bubbles or cracks can be seen. 
But I'm actually really curious, and want to prevent this from happening again!
Please leave your thoughts below so we can learn more from my mistake together. 
Did I start the drillium too early? Was there another solution?
TLDR;

Loose headset, and top cap bolt stuck.
T25 top cap bolt stripped.
Araldite in the bolt and on my T25 key, left it over 2 nights, didn't unscrew.
Top cap bolt is aluminium, compression plug bolt is steel.
Drilled off top cap, left an even more stuck top cap bolt in compression plug.
Drilled out top cap bolt, bolt seems perfect.
Please tell me why top cap bolt is even stuck in the first place.

Thanks @Michael and @Carel.
Guess drilling really helped in the end!