I'm not sure I'd the rules absolutely require an acrobatics roll for something like this. The PCs are invisible and so won't provoke an AoO, period, and the acrobatics roll is to avoid an AoO. So no matter what people think the rules are saying, we're well into GM adjudication and the right answer is the one that works best for you, your players, and the pacing of the game.
First - all turning invisible and trying to escape is a good plan, and may be a necessary one to prevent a TPK. My inclination is to not make it too hard to accomplish since they're already stacking the deck in their favor by turning invisible.
Second - The orcs won't be able to see and target them with AoO as long as they're invisible. But the PCs will want to avoid giving away their exact position or they'll likely just be in the orc soup again once the orcs adjust. So avoiding contact and avoiding excess noise are both important.
I'd have them make stealth rolls to determine how well their general movement is going to give them away to the orcs' perceptions. This will be important more for the follow-up rounds and how the orcs will adjust.
Then I'd have them roll acrobatics just to avoid making contact with wary orcs milling about their combat spaces, but I would cut the DC down from the CMD that normally sets the DC. The situations are different with the orcs significantly hampered relative to the normal case. Off hand, using the Acrobatics skill as a model, what's the difference between moving through an opponent's square and past them? It's 5. I'd set the base DC at 5 and then modify it by how fast they're moving. I wouldn't add +2 for every orc whose threatened spaces they move through. Most of those can't target them and are irrelevant and shouldn't add to the DC. I'd only add for each actual orc space they move through.
As far as failing, I'd take a look at how close they got to the DC. If they didn't even make the DC I set without any +2s for additional orcs, I'd say they bumped into the first one and halt movement. If they failed at some point but would have passed by just one, I'd prorate how far they got into the mass based on how close they got to the final and then I'd just displace the orcs they passed to carve out a space for them to start in for their next move action. Every orc in reach will almost certainly attack that space (with the 50% miss) but, as long as the PC keeps his head and doesn't strike back, he can try again next round, hopefully with a shorter path to escape.