Why isn't a depletion region considered at the P-N junction of a MOSFET?
How do enhancement mode mosfets even work: You are missing the creation of the "Inversion" layer. When you apply positive gate bias to an enhancement mode mosfet, the existing P channel near the gate contact starts to recede and you have your gate contact - depletion region - p substrate. If you keep going, the concentration of holes near the contact is so little, that you form, in affect a N region near the contact - A N channel now forms between the source and drain contacts. 

So at this point, is you examine a cross section of the Enhancement mode mosfet, from source to drain you will see: N+ - N - N+. If you now look "down" you will see Gate - N - depletion - P.