Well, one other way is by ignoring mournhold completely. Granted, this will require the guys and girls to discuss the alternate plan first and vote on it. But anyway, here is my reasoning, or Semir's if you want.
1: The legatus' plan was to put it mildly, simplistic. It is also rather obvious. While the boat was supposed to be subtle according to the guy, Semir expects the docks to be under observation. Now take our motley gang of mercenaries and have them board an unmarked boat. A small merchantmen or whatever the legatus thought appropriate and sneaky. Now have said boat leave the harbour at night. Boat, strange passengers boarded in the dark, plus it leaves at a weird time. Can you say suspicious? Hell, even if we leave along with a lot of other boats in the middle of the day, we'll still stand out. News of all those mercenaries at the party, and the lot that left after receiving letters from the
champion of Cyrodiil him/herself, will spread like wildfire. Yeah, that wasn't very subtle, not since people were talking about it.
2: As Semir already said, he doesn't like the prepared disguises. Sure, we know he can do disguises, but the disguise needs to be geared to the situation and level of preparation available. Doing a peasant or other low-class civilian is easy, mercenary likewise. Nobles or even middle-class merchants take more time, both in ways of appearance and the necessary documents. Pretending to be a member of a highly exclusive, elite order with a religious backbone takes months, at least. So in short, the robes might work. Ordinators? Yeah, Semir isn't going to place any bets on that one.
Ordinators are ridiculously visible and are kept to very strict standards by every level of the population, not the least of which being other Ordinators and the priesthood. And in Mournhold those standards are probably the highest. It is the capital after all. He wouldn't just need to learn the right words, but when to say them, in what tone, how fast or slow. How to walk, how to react to things and other people. To make it all seem natural. Experience can cut down on the amount of needed preparation, but he doesn't have that. Not disguising himself as an Ordinator or something similar. And this is just for the guy who has disguises as part of his job description.
Actually, we can go with the disguise plan. We just need to liberate some Bonemold or Chitin first. Those sets also come with face concealment, are military in nature which will help when meeting the Dunmeri army and they're used by less important folk.
3: We don't actually have information on how to contact the Mournhold informant. Ok, this is nitpicking.

Anyway, we could solve that by retconning one of our Blades into having the info. The more elegant solution would be to have it waiting on the boat.
4: By the time we reach Mournhold, the front will have moved. I also expect that we'll then have to go to said frontlines. So basically we'll be spending precious time doing catchup. Better perhaps is to go straight to said frontlines and try to infiltrate there. This is also where the most up to date information regarding the search can be found. I'm assuming the Dunmer didn't just decide to invade Argonia because last sunday had such nice weather. We could copy that information and use it for our search. The most risky option would be to wait till the Dunmer had found it, then run in and snatch it before they can whisk it away.
5: If the Argonians are the ones to find the loot, we'll have similar issues as with the Dunmer scenario, but it is less bad. The Argonians will be sufficiently provoked to want some payback. They'll be looking to counterattack and probably utilize the roots as an equalizer. So far, they've been doing poorly after all and they'd lose badly outside the swamps it seems. This gives us a larger amount of time to do the snatching than with the Dunmer scenario.