The conclusion I draw from that is they aren't keen on Ulfric winning is because then they'd have to deal with a problem sooner, while the Empire perservering means they have to deal with a much bigger problem later.
And Divide and Conquer... with all the animosity that the Thalmor's been brewing between the Empire, Skyrim, and Hammerfell, do you really think they'd come to each others' aid? They already divided and conquered off Eleswyr and Valenwood, and they convinced the Args in Blackmarsh to go off and do their own thing, wiping out Morrowind in the process while they were reeling from the Red Mountain eruption.
The point is that the "Divide and Conquer" argument is invalid here because the Thalmor lose either way.
Also try this approach(just to show how perspective work):
Stormcloak victory, open hostilities with the Dominion. Leads to small scale battles, draining Skyrims and the Dominions resources.
The Emprei is now in a state of peace and can focus on building its legions without having to worry about the civil war. Leads to the Empire growing stronger.
Since the Dominion is weakening Hammerfell decides to join the party and start a small scale war with the Dominion, draining both of them of resources.
This again leads to larger attacks on the Dominion, who is weakening. The Empire who have used all this time to bide their time now decides to denounce the WGC and joins with the sacking of the Dominion. Leads to the fall of the Second Aldmeri Dominion.
Skyrim and Hammerfell are weakened by their longtime wars with the Dominion and is re-conquered by the Empire who is now the nation with the strongest hand.
This is one way you can see the situation.