If they say that there are no flights after March, there probably aren't. So why not check with Iberia? They fly every day through Madrid. But a one way ticket will cost you almost as much as two ways. You should consider going both ways with them instead as that will save you a lot of money.
On the other hand, if you get a one-year-ticket from KLM, it seems to me like they should have some responsibility for getting you back home. But others probably know more than that than I do. I know for a fact that I got a one-year-ticket with an open return from KLM (for the same price as an ordinary three-month-ticket since I am under 26) in September and they didn't mention anything to me about not being able to fly me back after March.
As I mentioned, I'm not sure Iberia has flights for POP but from Santo Domingo you just catch the CaribeTours bus to Puerto Plata for around 100 pesos.
Air France is another option but I think that they are generally a bit more expensive.
I still think the best way to get some actual information on the price, is to call a travel agency in Sweden. They're typically very helpful and some are specaliced in the Caribbean. They all access the same database systems but some salespersons are better at finding good prices. Try Carribean Travels in Stockholm 08-30 21 03, Carribean Resor in Lund 046-12 75 34 or IncaTours in Malm? 040-664 50 80. I got my ticket from IncaTours, if you call them ask for Christian, he's very helpful.
Good Luck,
mne