In that case there will be no issue making US transfers an option. I think merging with option to transfer to US is best. It is impossible for the Dalborra to get any smaller even if everyone chooses US and if after that the pop is still too low then its time to think about shutting servers down.
There's tons of people on Dalborra. There's 128 people on Dromund Kaas alone right now. Maybe your pvp server is dead, but the other two are fine and growing. Why don't you just re-roll.