It looks like the Dremora and an Ogrim in service to Molag Bal rather than Dagon or Malacath. Have the Dremora traded allegiance again after so many defeats to Mehrunes? Insights?

http://www.elderscrollsonline.com/

Great resource to read short synopsis of the setting.

ESO takes place in the second era, several hundred years before Skyrim or Oblivion.

Elder scrolls online is set roughly one thousand years prior to the events of oblivion or skyrim. During this time period, tamriel is split into four regional 'governments' - The cyrodiilic empire, the Daggerfall covenant, the Ebonheart pact, and the Aldmeri Dominion.

Molag Bal, during the time period of ESO, is attempting to overthrow the world of mortals by 'pulling' nirn into his own plane of oblivion (known as coldharbour). He's doing this by having his cultists summon dark anchors - gateways in the sky which serve a similar role to the oblivion gates in Oblivion - allowing molag bal's daedric minions to travel into nirn, and over time merging nirn with coldharbour.

Molag Bal's cultists have also invaded and overthrown the cyrodiilic empire, and placed an usurper on the throne. This creates a two front war - the usurper, commanding the imperials, seems to be serving as a distraction for the other three alliances in order to keep them from interfering with Molag Bal's dark anchors. The alliances, in turn, are fighting both each other for a place on the throne, and against molag bal to prevent him from taking over nirn.

That about sum it up for you?

OP wanted to know why the minor Daedra are working for a different Daedric Prince than usual. http://www.uesp.net/wiki/Morrowind:Daedra#Ogrim

It's a good question, and I don't know the answer to that

Given we don't know much about the Daedra employed by the various Daedric Lords, it's entirely possible that Molag Bal has humanoid servants of his own. In fact, given how necromancy (especially vampires) is associated with him, it's very likely. Looking at the Aurorans, the Dark Seducers, the Golden Saints, the Knights of Order, etc. it's likely every Daedric Prince has at least a single humanoid race of Daedra following them.

Poor Mo needed to borrow his army from the other princes.

Not sure but I suppose we might find out...IN-GAME!!! Dun-dun-DUUUUUUN!