Answers

Earlier in the week I posted questions on Rashi.  In regard to the first question I found an answer in Likutaty Sichos volume 6.  The Rebbe explains that in third example of דבר הכתוב בהווה is different than the other places for in the example of טריפה בשדה the word בשדה is extra.  It is an additional חידוש to say the Torah will write an extra word to enhance the case and that Rashi can't find a source for in our parsha, and must look elsewhere.
The answer to question 2 can be found in Likutay Sichos volume 11.  He explains that there are two kinds of אם.  There is the אם used when the Torah is telling us when this event happens and that there are plenty of and isn't the rule of Rebbe Yishmoel.  Rebbe Yishmoel's rule is addressing when the Torah is describing the action of a person.  That's when he says its always a  רשות except for three places.  Therefore, אם יהיה היובל doesn't make the list.  However, Rashi in Vayikra is addressing a different issue than the other two places where אם means an obligation.  Rashi wants to give an example of where we may find אם meaning when and for that he cites a proof from יובל.  Why Rashi cites tthe name Rebbe Yishmoel is a little more complicated, you can see for yourself here.