The mention of acetal chemistry leads me to have some sympathy for your instructor, although this person is taking it to the extreme with the grading policy. Below are two mechanisms for a trans-acetalization reaction.
I am guessing that mechanism A is one that your instructor would give half credit (since it goes through a carbocation) and mechanism B is one that your instructor would give full credit. I prefer mechanism B, and I think it's fine for your instructor to agree. However, these are identical mechanisms. How is that? Because the third structure in each mechanism are the same thing, just different resonance structures.
So if they are identical mechanisms, why do I prefer mechanism B? Because that resonance structure explains something about the reaction, particularly in relation to a reaction that you have learned before, the SN1 reaction. In the typical SN1 reaction, formation of a carbocation is slow. However in the trans-acetalization reaction here (really just a variant of the SN1), that carbocation formation is much faster. Why? Because the carbocation formed is stabilized by the ability of the adjacent oxygen to donate electrons, giving an intermediate that has a resonance structure where all atoms obey the octet rule. I think that's an exceptionally important consideration, and one that helped me understand and recognize reactions of carbonyls.
I do think that it would be unfair to give different levels of credit for mechanisms A and B. However, I do appreciate what your instructor is trying to get across, although s/he may not be doing a good job at it.