I will echo other posters, you’re likely to see a 7AM EMH added that Easter Sunday. It’s not currently on the calendar, but I suspect it could be added. Last year, the two week period around Easter saw many MK AM EMHs. Right now the 2018 calendar shows a somewhat typical MK Friday AM and Wednesday PM EMH pattern around Easter, but keep an eye on that as it could change.
As to your other question, for 8AM regular park openings, they open the park at 8AM – not 7AM. The “Main Street open an hour before park opening” thing only applies to 9AM regular MK openings. For 8AM or 7AM regular park openings, the park opens at that time or a few minutes (0-15ish) before.