Standing for birchas hamitzva and the bracha for hafrashas challah

There is an interesting machlokes whether the bracha on hafrashas challah is l’hafrish challah, l’hafrish terumah, or l’hafrish terumas challah. Part of the issue revolves around the different terms for the mitzvah: the Torah calls the separation terumah, yet the Mishna uses the term challah. The Aruch haShulchan paskens to say l’hafrish challah based on the BH”G’s use of this expression. My wife follows the custom of saying l’hafrish challah min haisa, but you should only use this nusach if you are separating challah from dough and not after baking.
The Shulchan Aruch in hil tzitzis (siman 8) writes that a birchas hamitzva should be recited while standing. The Magen Avraham asks how is it that women were mafrish challah while sitting (as is clear from a number of mishnayos in perek 2 of challah) – weren’t they required to stand for the recitation of the birchas hamitzva? The MG”A answers that the rule of standing for birchas hamitzva does not apply to mitzvos like challah or shechita. What is the sevara to distinguish? I would have said that these mitzvos are different because there is no requirement to make dough for challah or to eat meat – if one chooses to do so there is a kiyum mitzvah, but there is no chiyuv. However, why then should one have to stand for the bracha on tzitzis when m’ikkar hadin one is not obligated to wear a four cornered garment? The Pri Megadim offers a different sevara: the MG”A means that challah and shechita are not true mitzvos aseh, but are issurei aseh – meaning, the mitzvah of challah functions like an issur of eating dough without hafrasha, and the mitzvah of shechita functions like an issur of eating meat without proper shechita. Apparently the nature of mitzvas tzitzis is not an issur aseh of donning a 4 cornered garment without tzitzis, but a positive mitzvah to fix tzitzis to the garment (see Tosfos Yevamos 90b). Maybe more on this later...