To remove the hard drive you remove the coin screw on the bottom and then pull the hard drive caddy out to the side. No need to open the laptop.
It sounds like maybe you figured that out but it's unclear.

Try hooking it up to an external monitor. You might need to toggle a keyboard combination to get it to display on the monitor, and I don't remember what that key combo is. Fn+F5? or something like that. If I remember correctly, the F-key you want will have a blue icon showing a rectangle (supposed to look like a monitor). The toggle has 3 states - built-in LCD, external monitor, and both.

Hopefully you're using a real first-party IBM power adapter and not a generic knockoff - those can cause problems and could certainly keep it from booting. If you're not sure, post a photo or 2 of it, including the label.
If it's not a real IBM adapter then replace it - and be careful when shopping because last I was aware, 95% of the Thinkpad power bricks on eBay were knockoffs. If it was sold as new, it's a knockoff for sure.

If it is legit, then you can do a basic test with a multimeter to check what voltage is coming out of the barrel connector. Put positive inside the barrel and negative on the outside (don't arc them). I don't remember what the correct voltage is but it's marked on the brick. It's a regulated power supply so if it's not at the correct voltage then it's bad.
This isn't as good as testing with a load but it's better than nothing.
Testing with a load would be more involved, to do that you'd probably need to crack open the power brick and measure at the output of the brick while it's powering the laptop.

Can a bad (for instance chinese) ac adapter cause serious hardware damage?...

https://ibb.co/0tKZx3R

You won't believe it.. just for fun I wanted to test the notebook again... I put it on another desk, so I have to rearrange the cable of the ac adapter... And: it boots!

Lazar81 wrote on 2020-12-13, 09:42:

Can a bad (for instance chinese) ac adapter cause serious hardware damage?...

https://ibb.co/0tKZx3R

I believe that it could. Cheaply built PSUs can subject the machine to excessive ripple voltage which can damage ICs.
I had a knockoff PSU for a similar Thinkpad several years ago and for the few days I used it there were stability problems.