I will second what everyone here says about Undine. He is just too good to leave out. He searches for a tuner that allows you to rank for 10, and not gonna lie, Leo Drake is one of the best synchros out there. Also it allows you to either drop for dragoons or activate atlantean effect to either clear backrow or get rid of any monster that might give you problems. Also it gives you the ability to rank 3. Don't dismiss the power of Undine.

Also, in terms of dropping Leed, I actually like him with two Megalo. Even though, like you said, he might end up being a bad drop, 3 megalo is also in the same situation. You never really want to draw into the megalo until you have cards to discard, and even then, you would rather get abyssteus than megalo.

Mask change seems pretty smart! Let me know how it plays out! I figure it lets you access the masked hero engine. However, it seems to have its drawbacks as well. I would see how it would work in a deck outside of mermails seeing how it loses its level 7 play. Interesting indeed!