What are some of the ways Daedra end up in Mundus? I know that they can be summoned and bound (Conjuration), and I know that they come in force via gates (Gates to Oblivion), but are there any Daedra that come to Mundus jus because? Or maybe by mistake? Or are there other ways to make them come?

not that I know...

There may be some leftovers from the Mythic era before the boundaries were up... they would exist only in remote places, or mastering Illusion may hide among mortals to this day. This is only hypothesis.

Two words: Blueberry pies

None that I'm aware off. That's that that barrier is for.

I was just wondering, When the Barrier was fully open for a few moments, I guess there would came quite some Daedra to Nirn, but would they be banished back to Oblivion after the barriers were closed forever.. (or at leastfor now) If not that would explain it if their would be wild daedra in TES V, no mather where it takes place....

What I heard was that the leftover daedras were all svcked into the sloadmade reverse-reconfiguration-cannon in Thras, ending up in small bottles. Popular to give the younger ones as toys but not much use really.

Wizards can summon Daedra on a more permament basis (if you can control them). Witness the scamp you had to kill for... I believe House Redoran. The one in that apprentice wizard's house.

Which also means if you are a noob at conjuring and especially binding daedra, expect your "servant" to serve you a giant mess in your home or your own beating heart.

I think that's a Mages Guild quest

We must keep in mind, of course, that the little incident near Maar Gan happened before the crisis concerning the Oblivion Gates, and the strengthening of the barriers between Nirn and Oblivion.