I've played this many times before but never went back to the old mine after it's collapse. Does anyone find it strange that one can enter the collapsed mine and fight all the guards, etc?

I did not know one could do that - and, to be honest, I can't.
For me, the entrance to the old mine is barred by an iron grille.

I thought that'd be the case since one of the describers (Diego or Milton) said the guards sealed the entrance but, alas, it was open and I was attacked as soon as I entered to have a look. Diggers were still digging. :)

I just noticed the discontinuity in the ending too - the hero states that he returned to the surface and the barrier was gone. However, in II, he was buried alive.

I always disliked that retcon about Gothic's ending in the sequel. I understand that they wanted to make the main character weak again, but it is a silly way to do it in my opinion.

As for ability to enter the Old Mine after its collapse, I would assume it is just a minor bug.

As per the mine, I think you are correct. I'm playing through again - I'll check but I'm betting the gate will be closed this time.

Yeah, several ideas came to mind on how it could've been done. It was really blown on the II-III transission too. "he's a little rusty" :) They already had a ready-made story line since the ship was overburderned at the end of II. Shipwrecked and the hero found nearly dead on shore and taken in by someone in their humble hut.

It's still able to be entered on this palythrough so, at least we know it isn't a random bug.