He's not in the Inn at Ald'ruhn. I've asked the NPCs about his whereabouts and they claim he's downstairs, but he's nowhere to be found. What's going on?

he should be there. there are two ways to get downstairs. you need to upstairs and then find the down stairs.

Is he in a different cell altogether? I have a feeling the main quest is now broken because he's nowhere in site. Is there a console command in which I can summon him in front of me?

There are 2 different downstairs areas inside inn, have you checked them both?

I'm not sure. I searched everywhere downstairs but it's not that big. There are 2 NPCs and a guard and two rows of beds on opposite sides with a few tables directly below the bar. I haven't found any doors that lead to other cells. I also used the "ResetActors" command to see if it helps.

As you look towards the barkeep on the top level, there is another set of stairs to your right. Go down there. He's hangin' with a Bosmer and a Breton.

Well, those areas are accessed by 2 different sets of stairs. One is closer to the main entrance, the other is farther in the bar area - that's what Knightly probably meant saying that you need to go upstairs and then find the down stairs.

Thanks a lot! I can now continue the main quest. It's truly getting more and more exciting now that I got into it.

In future, use

TWF

in interior cells to locate hard to find NPC's. If you don't feel like cheating, Morrowind Code Patch mods Detect Animal spell effect so that your map marks NPS as well.

EDIT: Spelling madnesssssssssss