tzitzis worn without kavanah - lack of kiyum mitzvah or bitul aseh?

The M.B. in Biur Halacha (siman 60) says a big chiddush: if a person puts on tzitzis but fails to have kavanah to be yotzei the mitzvah, they are mevateil a mitzvas aseh.  Therefore, he writes, a person needs to be careful when they are called up for an aliyah and they grab a talis, to make sure to have kavanah l'shem mitzvas tzitzis.  (He then backtracks a bit and suggests that since the talis in this case is being worn momentarily only for the sake of getting an aliya, it may not really be a "levisha" which is chayav in tzitzis.)

I saw some assert that this chiddush contradicts a Minchas Chinuch.  The gemara darshens that "v'asu LAHEM tzitzis" excludes using a stolen item for the mitzvah.  Why do we need a derasha to give us this new din when we already have a principle of mitzvah ha'baah b'aveira?  Minchas Chinuch answers that mitzvah ha'baah b'aveira negates the kiyum mitzvah.  It is as if the person did nothing, and therefore gets no credit.  The derasha goes a step further and teaches us that using stolen goods results in a bitul aseh.  It's worse than doing nothing -- it's like wearing a 4 cornered garment with no tzitzis.  (M.C. says the same chiddush by sukkah where there is a derasha to pasul a stolen sukkah on top of the din of mitzvah ha'baah b'aveira.)  

According to MB, failure to get credit for the kiyum mitzvah, e.g. where there is a lack of kavanah, results automatically in a bitul aseh.

R' Elchanan in Koveitz Shiurim (Kesubos 250) questions whether tzitzis worn without kavanah is doche kilayim.  This safeik should hinge in part on whether you accept the chiddush of the MB.  If a lack of kavanah is equivalent to being mevateil the mitzvah, there is no aseh to be doche the lav of kilayim.  According to MC, however, the gavra may get no credit for the mitzvah, but l'maaseh he wearing a garment with tzitzis, and perhaps the issur of kilayim does not apply to a garment that has proper tzitzis on it.