Let me take the second part of your question first. If the door doesn't have a lock, it's treated as if it does have a lock and indeed that said lock is locked, insofar as opening the door is concerned. That seems, to me, to be the gist of the spell.
What if it is locked and somebody has a key? Then I'd say that it doesn't matter that someone has a key. It's not simply the door's lock that prevents the door from opening normally; it's also the spell effect that holds it shut, as firmly as if it were locked.
By this reasoning, somebody could dispel the hold portal and find themselves faced with a locked door, assuming it was locked in the first place. On the other hand, if the spell isn't dispelled and someone with Open Locks tries to use that skill to open the door, it doesn't work. A successful use of the skill may unlock the door (again, assuming it was locked in the first place, of course) but the door remains firmly held by the spell effect, for the duration of the spell.
What I'm not sure about, now that I look at the wording of the spell, is this: what happens to a locked, magically held door that has the knock spell cast on it? If knock negates the hold portal, does it also unlock the door? Two results for the price of one spell? A 2nd level spell being more useful than a 3rd level one? Well, I'd rule that knock negates the effect of hold portal and unlocks the door. Yes, it's getting a big bang for your buck but it's a very limited circumstance.
