The Gemoro on 3b is trying to find a case of Bal TeAcher with Nezirus, and suggests that someone says he will become a Nozir at some point before he dies. Since he does not know when he will die, he must become a Nozir immediately to reduce the risk of transgressing his vow (and if he does not he transgresses Bal TeAcher). [I think the RAN explains his statement is not itself an acceptance of Nezirus and he must accept Nezirus immediately, but others disagree]
1) The RAN mentions another case (if I understand him correctly) that a person vowed to become a Nozir within two years; in this case he would transgress Bal TeAcher after three Regolim. Is this correct? (His vow was not necessarily to start until 2 years are up!)
2) What would be if a person says "I will become a Nozir within the next two weeks" - would he transgress Bal TeAcher after two weeks if he has not started Nezirus? If so, isn't this a slightly easier case than the Gemoro's answer?
1) The RAN holds that when one takes upon himself to accept Nezirus, it is like a Neder of a Korban, since we learn it from a Hekesh. Therefore Bal Te'acher applies. However, since, generally, Nezirus takes effect immediately with the Kabalah, it is difficult to find a case where it should apply. With the case where he accepted to observe Nezirus only within two years, the Ran has found a case where the Bal Te'acher will be the same as Korbanos. As you pointed out, since his Neder applies only within two years, Bal Yachel will not apply until two years pass. Nevertheless, the new Isur of Bal Te'acher which is learned from Korbanos will apply after three Regalim.
2) If he says that he wants to be a Nazir within two weeks, although he will transgress Bal Yachel after two weeks, he still will not transgress Bal Te'acher. The Ran says that there is a special Bal Te'acher immediately when he says, "Lo Epater Min ha'Olam..." since it is implied that he wants to be sure to have the Nezirus take effect, and therefore he must do it immediately. However, if he gives a time limit there is no implication that he must rush.