I wonder what the point of taking Danny back to the Dothraki is? It can't be for her to get the Dothraki to be her army to invade Westeros. Tyrion in fact points out to her in their meeting that she doesn't have much support in Westeros. Their fathers combined destroyed one of the houses they could have compelled to support them, The Starks. Dorne, which would be the obvious landing point for an invasion is thought to be weak militarily but politically aligned with her. And the rest of the houses have no reason to support her. Danny also wants the support of the small folk. The Dothraki horde can not be kept in check. Once lose on Westeros they will rape and pillage everything in their way. That's their way/culture. You can't expect the support of the small folk if that happens. I'm really scratching my head as to where they're going with the Dothraki storyline again. Also now that the fleet is in ashes the Iron Isladers/Euron is the next logical way for them to finally get Danny to Westeros. The way I see it by seasons end Danny needs to have an army, a fleet and be on her way to Westeros. I think the biggest reason that TWOW hasn't come out yet is because RR is having a hard time getting the chess pieces to line up where they need to be for ADOS.
I can envision a reason for her to get her own personal horde.
Your scenario essentially assumes she will abandon everyone on the "mainland" and only concern herself with Westeros.
There are plenty of other power blocks on the mainland that are based on slavery, and she could easily keep the Dothraki busy attacking them.
This would protect her rear and support the slaves she has liberated so far.
If she doesn't do something about those other power blocks, the moment she sails, those people are toast.
Also, I don't see the Dothraki being keen on getting on ships anyway. They are horsemen first and last.
Wasn't there a line to that effect in an earlier season?
So if Danny recruits the Dothraki, that let's her leave all of them on the mainland, "protecting" the freed slaves, while she takes all her other allies to Westeros.
As far as how she get support on Westeros, she hooks up with Brant and thus gains the name of the Starks to support her.
They've already implied that with Bolton talking to Ramsay about how important Sansa was to their plans.
So that gives Danny the North, and once she and Brant smite the White Walkers, the rest of Westeros shouts Huzzah! and they all lived happily ever after.
Of course, if all of this is wrong, it could simply be a way to give Mormont an honorable death.
Since he's got the "stone skin" disease, or whatever it's called, he's toast, grated cheese.
So if this is simply the way they let him exit, stage left, I'm OK with that.