Well then I suppose I ought to say that I don't think a rookie will head north in April.
I guess I should also say that, because of service time games allowing an extra year of team control for a few weeks in the minors, Fernando Romero in particular absolutely should not head north in April.
I'm a bit on the fence but I'm leaning towards disagreeing with your second point. Presumably any rookie would be up with the intention of being sent back down when Santana returns. That leads to four likely outcomes.
First, Romero is pitching poorly and when Santana returns and he is sent back to the minors. This extends his service time another year.
Second, Romero is pitching ok but is still sent down when Santana returns thus extending his control.
Third, Romero is pitching phenomenally and another pitcher leaves the rotation instead. He could still be sent down at a later point in time or even in a future season to regain that seventh year of control.
Fourth, Romero gets injured and accrues MLB service time. If it is a short term injury then the first three outcomes still apply. If it is a major injury then the likelihood of Romero being the impact pitcher we are all hoping for is greatly reduced making the concern over service time, not moot, but certainly lessened.
The only scenario where service time is lost is if he is pitching amazing in which case, I'm glad they brought him up anyways. The Twins are in contention and the time is now!