Just a few thoughts:
- I remember seeing this topic discussed on the worldwar2.ro forum some time ago. I don't remember the details very well, but it was said that the Romanian army only had ammo for a few days (or even a few hours, my memory is hazy). The point was that there was not enough ammo for an effective resistance.
- The ultimatum was on 28 June, after the end of the Battle of France, so if Hitler wants to get involved, the Wehrmacht has nothing better to do anyway.
- There is a Romanian historian (Neagu Djuvara) who has mentioned this scenario several times in his books. He claims that Romania should have resisted, and if things went bad in Bessarabia, the Romanians should have convinced Hitler to occupy the rest of the country so Stalin does not take it all. His overall point was that fighting in Barbarossa was futile because it accomplished nothing in the end, and that Romania would be better off if Germany and the SU would have divided it among themselves in 1940, just like Poland. After the war hopefully the country would be restored by the allies, without the loss of international prestige caused by being an axis country, and without the losses of the war.
- if the whole thing escalates into an early Soviet-German confrontation, I think the Germans should be slightly favored, and the Soviets loose some of their advantages.