1. If one intends to wear multiple pairs of tzitzis one should recite a bracha only on the first and then put on the other garments. The Rama (O.C. 8:12) writes that if a person took off the first garment before putting on the second garment a new bracha must be recited on the second garment. Many of the Achronim disagree. Since the person had in mind that the bracha would apply to all the garments, what difference does it make if the first is taken off before putting on the second? The Magen Avraham asks how this case is different than a person who recites one bracha intending to shecht multiple animals -- at the time of shechita on the second animal, the person is already finished with the first, yet no new bracha is required?
Seems to me that there is a clear difference between these cases. In the case of shechita, the mitzvah is finished m'meila once the first animal is done. Even if the person wanted the mitzvah to continue, it can't! In the case of tzitzis, it is only the person's deliberate removal of the garment that creates an interruption. If not for that, the chiyuv tzitzis accomplished by wearing the first garment would be ongoing. It's that choice to deliberately end the mitzvah which creates the hefsek.
2. The Beis Yosef (and GR"A) paskens (O.C. 8:14) that if you remove your talis, even if you have in mind to put it back on immediately, a new bracha is recited. The MG"A asks how this case is different from the case of a person who leaves a sukkah with the intent to return, in which case the Beis Yosef himself paskens no new bracha is required. Interestingly, here the Aruch haShulchan draws a distinction similar to the one I drew above -- there is nothing compelling the removal of the talis, so the choice to do so creates a hefsek. However, I think a more basic distinction can be drawn between the cases. The mitzvah of sukkah is one of dirah, living in the sukkah just as one would live in one's home. Just as living in a home does not mean imprisonment -- one can pop in and out as need be -- so too, by definition the mitzvah of sukkah allows one to pop in and out as need be without creating a hefsek. The mitzvah of tzitzis, however, does not allow for interruption -- take the garment off and the mitzvah ends.
3. I am not sure what to make of R' Chaim Volozhiner's statement in Keser Rosh that tzitzis have to be down to your knees.