Obviously all speculation until we see how DHS DAH will "feel" / "operate" for guests, but my two cents/guesses....
I think DHS EMM is more than a few hundred people, but that's for another thread. I think in comparing the two, the time difference is key. 1.5 hours (or less) vs 3 hours. Day time vs nighttime. Unless DAH turns out to be a totally overcrowded affair and everyone spends their time in TSL, I think I'd rather go with DAH. EMM (which I like by the way and will be doing again on future trips) has a bit of "clock watching" to it since you're trying to maximize limited time. I'd rather have longer event time, as I'm assuming SDD is going to be a bit like 7DMT and always have a bit of a wait all night.
I think the first night of anything 'can' be subject to Disney calling out the media, but it's the whole park vs a private event in a small/contained area like Tony's/the viewing area, so different set of circumstances. I'm inclined to think it will be a non-factor. Also Disney doesn't always do media events on the first night of something, they can be later sometimes.
Day guest "carryover" into DAH time has always been a component of the MK DAHs, although does not often get mentioned as a particularly negative part of the event. It mostly impacts rides like 7DMT and PP, and maybe a few other slow loaders (speedway)... but other than that a lot of MK ride lines can tend to be pretty darn low/empty by closing time naturally. But, most people do recommend hitting the lower tier/lower priority rides first, then circle back to things like 7DMT and PP later (both of which still usually maintain some level of wait during DAH).
I'm curious to see how DHS and AK will go since there are fewer "lower tier/lower priority" rides to absorb DAH people at the start of the event. The current (but subject to change) park calendar which has 8pm closings and 8:30pm DAH start time could be a big benefit for ride clearing (but we need to see if those hours will remain the case or not).
Another thing that gets sporadically reported at MK DAH is guests being allowed in the FP line once DAH starts (bypassing any leftover standby line). This doesn't seem to be consistent across rides/nights, so I think it has been a CM/YMMV thing. TBD if any sort of setup like that is used at DHS.
Sorry, long post. A lot we don't know and need to

to see what happens.