Can you build accurate models using 100% LEGO elements by following Megablocks instructions?
Not necessarily.

It really depends on what you mean by "accurate."  Most (probably 90% or more) of Mega Bloks elements have identical, or at least functionally identical, LEGO elements.  However, there are a number of Mega Bloks elements that you can only approximate with LEGO elements.  For example, Mega Bloks hinges rotate around the center of mass through 100+ degrees center at the vertical, whereas LEGO hinges rotate through 90 degrees from horizontal to vertical.  Also, Mega Bloks has a SNOT piece with studs on both sides that LEGO unfortunately does not have.

If you are willing to slightly modify the instructions and accept a slightly different model, then yeah, you can reproduce anything made by Mega Bloks using LEGO elements.  But if you want the models to be 100% accurate, then no, not all Mega Bloks sets can thusly be reproduced.