I think I have come up with all the connections between MH and EAH with what we saw in 13 Wishes and from EAH's history.
1.) The Two Sisters- I think this is where everything began. My theory here is that the sisters found Gigi's lamp and shared the wishes. The two are also said to have fled the EAH world through a portal which was located in a wishing well. I believe that they wished for a portal to open and then Giles secretly influenced one of them to wish for Wisp's creation. When the sisters fled, the lamp decided to go with them for some reason.
2.) The Deep End of the Pool at MH- I think the deep end is hiding the portal the Two Sisters ran into to escape their world. It is said no one knows what is at the bottom of the pool or where it goes but it makes sense that it hides a passageway to EAH. I think that other portals exist but the one in the pool is the most secretive one of all.
3.) The Mirror in Headmaster Grimm's office- At the end of one of the first episodes of EAH, we saw a mirror remarkable similar to the one Gigi used to protect Fawn. I think the whole Legacy Day signing refusal coincides with 13 Wishes before Wisp reformed. You can clearly hear her laugh from the mirror and I think I saw for a split second a shadowy silhouette of her in the mirror.
4.) Madeline Hatter, Kitty Cheshire, and Lizzy Heart can see and hear other realms and so they are fully aware that another world other than their own exists. The Mirror Cupid used to cross over I believe was from Wonderland and has dimensional teleportation powers that draw from Wonderland energy.
wow, never thought about that................
Saw the Thronecoming episode last night and I was wrong about the mirror part but I'm pretty sure the other theories I have are true.