2 Questions on Ultramarines

In my mind, you can as long as you tell your opponent that you are not playing dark angels.
For the HG question, command squads can be used as honour guard squads. I know quite a few people who have done that.

HPA

You can use whichever models you want to represent whomever you want as longs as:

1. Its follows WYSIWYG

and

2. It bears resemblance to the model type it should be(no hive tyrants for commanders).

I use one too

I also play Ultramarines and I too have a chaplain in a man-dress. I just highlighted his robe with Ultramarines Blue, and he fits right in with my hard-chargers!! The model I use also is holding a plasma pistol. THe people I play with do not press the WYSIWYG issue so no one minds. Sometimes they ask, (fearfully, I might add!) "Does that Chaplain really have a plasma pistol?" I just say nope and we play on. I have not played in a sanctioned tournament which enforced the WYSIWYG rule, but I guess if I ever do, I will have to pay for his plasma pistol, but I can still use him in my army!

uurrgghhhh SMURFS all should be purged by flame!.

nothing illegal as long as it's wysiwyg, although i'm sick of facing ultramarines and black templars, People please for all of us non-marine players sanity change it up a bit!

I use Ezekiel in my DIY chapter as my librarian (when not fielding my terminator librarian). As others have said, as long as he's modeled showing the wargear you've paid for, there's no reason you can't use him.