when there are extra magic hours, what mountains aren't open? Will they let you line up a few minutes before they open?
When there is AM EMH at MK only Fantasyland and Tomorrowland are open So, Space Mountain and 7DMT are open, but Splash and Thunder Mountain are not.
Until the regular park opening time, guests are held out of the Adentureland/Frontierland/ Liberty Square parts of the park by ropes near the Adventureland arch, the entrance from the hub to Liberty Square, and where Fantasyland transitions into Liberty Square (near the Tangled restroom area). At opening guests are let into the rest of the park, but are usually walked in by cast members so the three groups arrive at the Splash/Thunder area at the same time.
In our experience, that first wave of guests creates an immediate line at BTMRR (moreso than Splash) but as a PP said, we still prefer going to BTMRR first because that line moves pretty quickly and the ride is pretty short. When we go right from BTMRR to Splash, it is still pretty much a walk on. Because Splash is a longer ride, the line at BTMRR may be a little longer (but still usually manageable) when you get off if you elect to do Splash first.
My favorite approach to that part of the park if we go there right at opening after EMH is to do my five favorite rides in this order:
BTMRR
Splash
Jungle Cruise (because the line there builds up faster)
Pirates
Haunted Mansion
When we do this, we can experience these 5 attractions in about 2 hours or a little less. When we get to Jungle Cruise the posted time is often 20 or 30 minutes but we get in line anyway because the wait is usually not that long and we have a pretty good idea from the number of people in line if it will be that long.
When we go back to Pirates the posted and actual wait times are usually about 10 minutes, and at Haunted Mansion our actual wait is not more than 15 minutes or so, even if the posted time is a little higher.
This plan has worked well for us many times, including on the Friday after Easter (Star Wars Dark Side Weekend) with an 8 AM EMH and 9 AM park opening.