It's not a long commitment for Allen (they film mostly in summer, a part like Theon represents a few days of filming), it wouldn't be much of a problem getting him back in two years (no harder than keeping him until then will be). Actors are quite used to that, and GOT have this big advantage over an original story that they already know Theon will have a substantial part in season 5.

That's more like it. Putting Theon and the Greyjoys on hiatus for two seasons would have been a waste of all the efforts they put into building them up in season 2. They can wrap finished arcs, but if it's ongoing they need to keep it ongoing on a TV show.

That's one of the challenges of adapting something like ASOIAF for TV. They can't afford to have important players or arcs vanish for very long periods of time as it's happened in the books. If they aren't seen, they need to be discussed (as will no doubt soon start with the Freys) or the audience will either forget the details of their arcs (wasting screen time refreshing their memories about Theon, the Greyjoys and exposition about what happened to him in the interim) or wonder where they've gone. The torture scene in 3.02 did just that: it cleverly reminded the viewers of Theon's father/people and his sister, what he did in season 2 and why, beside setting up the arc they'll give Theon this season and creating an intentional confusion/puzzlement about who holds him, which they also fed with the different news about events North and the fate of his brothers Robb got.

Another motive behind a Theon arc, I suspect, is that it provides them with a good and clever way to build toward the conspiracy aspect of the Red Wedding, while providing the audience with good drama and - we'll see how long it lasts - with some mystery around who Theon's captors are (the character was also a favorite of the viewers in season 2). They'll want the average viewer to understand the plot twist and the motives behind it (especially since the victims won't be around anymore to provide any exposition.).

Exactly. They also inflated the part of Robb's love interest, for dramatic purposes (for now she seems mostly there to increase the drama and emotions in this arc and make Robb falling in love more tangible, and thus makes it easier to build this aspect later. We'll see later if they did or not make her a Lannister agent too) and turned bit players into recurring ones, like the whore Rose. And they also revealed the identity of that King's Guard who joined Daenerys very early (though it's partly so the viewers don't have time to forget him and partly because it's not a book and unlike Daenerys we know that guy on sight.

I think it's more likely they postponed his introduction to an episode where he can have a proper one. It's spreading the new stuff in Arya's arc over a few episodes, and limiting the number of speaking parts when possible is a good way to stretch their budget (and to help the audience keep up with the new players).

They did much the same with the Reeds. They didn't appear last season and people thought they had been cut, but they just waited until they really needed them and they could give them a good introduction, which was season 3 (clever casting, they help make Bran appear younger). They already had Thoros and the Brotherhood, the old Lady Tyrell and the Reeds to introduce in ep. 2, my guess is they're waiting to introduce Dondarrion.