Stuck Compression Plug and Top Cap Screw
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!