Culhwch: you memory is a little faulty, I think.
Maege Mormont was with Galbart Glover on the way to Greywater Watch, but she has no real force with her: they took only two longships. She might get to Greywater Watch, but the crannogmen are no army. They're guerrilla fighters at best.
Also, many of the Riverlords are hostage - or have switched their allegiance to the Lannisters. Lords Bracken, Mooton, Piper, Vance and Goodbrook all bent the knee. Meanwhile, the Blackfish has only enough men to guard Riverrun. Besides, he is besieged there by Lannister forces. So there's no prospect of relief from the Riverlands.
As for Frey, well, it's Littlefinger who's Lord Paramount of the Trident - technically, the Riverlords answer to him. Of course, he may have trouble enforcing that.
Jeroam: had to check this before I got back to you. GRRM has been cagey about how many men the Ironborn can field, but the best estimate is that they can field about half what Dorne, the North, the Lannisters, or the Riverlords can field, slightly less - about 20,000 men, almost all foot.
Now, since 20,000 of the Northmen's host at least have been scattered or killed, that means the Ironborn can probably control large chunks of the North. Bolton will certainly have his problems: he can probably raise no more than 10,000 loyal men at present, with about the same number of Stark loyalists like the Umbers, Manderlys and Mormonts, though the latter group will be scattered and won't have a single commander.
For the rest of Westeros, though, the Lannisters and Tyrells have fully 100,000 men under arms. The Ironborn can't compete.