Let me start with your second question. Bear in mind that

a. the urge on the part Ya'akov and Eisav to leave their mother's womb prematurely was an abnormal occurrence, and

b. that unborn fetuses do not have freewill and choice.

I would therefore suggest that the events that took place at that time (however one interprets them) were Divinely inspired (and not of the babies' own choosing). They were in effect, a prophecy, to let Rivkah know the nature of the twins to whom she would give birth (as the Pesukim that follow indicate).

Now let me tackle your first question: The difficulty only applies according to Rashi, who explains 'le'Da'tan' to mean with their consent (just as you indeed said). According to the Aruch (cited in Chulin, at the side of Tosfos DH 'Al Tikri'), 'le'Da'tan' means that they were created with developed minds (and not with the minds of children), there is nothing to ask.

Seeing as the principle 'No'ach Lo le'Adam she'Lo Nivra Yoser mishe'Nivra' refers to the facts, and not to what a person may think, perhaps it will be more appropriate to ask from the Mishnah in Pirkei Avos (at the end of the fourth Perek). The Tana there states that a person is born against his will, which the Bartenura explains to mean that when the time comes to be born, the fetus does not want to leave its mother's womb (see Tos. Yom Tov). This certainly appears to be in direct conflict with the Gemara in Rosh Hashanah.

The reluctance to be born referred to in the Mishnah in Pirkei Avos can be ascribed to two things:

1. to the comfort that it enjoys in its current location, as opposed the trials and tribulations that await it in the world that it is about to enter (above all the battles it will have to fight to make a living), and

2. to the spiritual challenge that it will have to face in the form of the Yetzer ha'Ra (that will enter him immediately after he is born), a challenge that most people fail to live up to.

Either way, the unborn fetus is afraid of this and would prefer to remain where he is. Our Gemara, on the other hand, is speaking about the time of creation, before the Yetzer ha'Ra had entered man, when the prospect that an 'unborn fetus' faced was life in Gan Eden, with the snake as his servant. Serving Hash-m under those circumstances was something to look forward to. No wonder man was happy to be created.

Another possible answer may well be that one cannot compare an unborn fetus, who sees only his current sheltered life, from the point of view of a fetus, with a Neshamah, after Hash-m has explained to it the advantages of life in this world and the potential reward that awaits it in the World to Come.

Be'Virchas Kol Tuv

Eliezer Chrysler