Matzah lishma - omeid al gabav

Matzas mitzvah must have "shimur l'shem matzah", with various opnions in rishonim from when that shimur must be done. If matzah is baked by an aku"m who is supervised by a yisrael omeid al gabav, most rishonim hold that this is invalid, with the notable exception of Rav Hai Gaon. What is the issue here? There are actually two possible ways to approach the machlokes. By get, aku"m with yisrael omeid al gabav does not work because we assume that the aku"m will break off concentration before finishing the writing of the toreif. However, the Rambam and Rosh argue about whether this would work for the process of tanning hides for tefillin where the extent of lishma is placing the raw hide in the tanning vat. We might argue that baking matzah is parallel to tanning the hides. However, this is not such a simple statement to make - the Noda B'Yehudah notes that lishma is required for the duration of the kneading process, which is far longer than the tanning process (see Minchas Chinuch). The Netziv in his commentary to the She'iltos (P' Tzav) takes a completely different approach. Instead of assuming a constant bar for lishma and questioning whether omeid al gabav meets that criteria, he suggests that the issue at hand is the nature of shimur itself - is shimur a chovas hagavra of the baker, or is shimur a din in the cheftza shel matzah, i.e. the dough must have the potential to become chameitz and therefore require some supervision to obviate that danger. (This requires some twists in learning pesachim 38 which you have to have a look at in inside for).