Maybe don't evict patrons who choose not to pay the extra price for nighttime shows, as that would certainly appear harsh as you pointed out in that example. Asking people to leave, then return later would create too much of a backlash, and be more hassle for everyone involved than it's worth (not to mention appear sorta embarrassing for those having to temporarily leave park grounds).
A simpler option could be to have enclosed holding areas inside the park where the nighttime fireworks / shows cannot be seen by those guests who do not purchase the extra ticket. Then once the show concludes, allow them back out into the park amongst other patrons to enjoy the remainder of their evening at leisure. Have a certain time scheduled (half-hour prior to any nighttime shows) where they must arrive at the closed-off location to await in holding while the performances run their course. This would avoid having to cut their nights short due to not purchasing the extra admission for the entertainment.
Another idea, since this extra ticketed nighttime proposal isn't as popular as expected, why not offer discounted admission for those who choose to purchase the extra ticket, yet make the viewing of nighttime shows optional despite whether or not they pay the extra $$$? I know that sounds kinda wordy, but here is how it would pretty much work out:
Let's say someone is purchasing a one-day MK ticket for $105. This person does not want to purchase the extra nighttime magic ticket, so he/she just pays the $105 (+tax) and let that be that. Since this guest did not purchase the extra ticket for nighttime entertainment, then it is optional whether or not he/she stays for the fireworks, castle projection show, etc. If they stay and watch the fireworks, no problem. If they leave early and miss the fireworks, no problem. Their day is paid for either way.
But now let's say someone is purchasing a one-day MK ticket who chooses to buy an extra nighttime magic ticket. In this case, their regular admission would be discounted some, say to about $75 approx. They purchase their extra nighttime entertainment ticket along with it, which realistically would cost no more than $30 considering it's not worth something up to caliber of MNSSHP or MVMCP.
So they pay the discounted $75 for their one-day admission, plus the $30 for nighttime entertainment, and voila! they now have their full day & nighttime entertainment ready to go!
Not saying any of these various ideas would work, or are even necessarily good ideas. But just thoughts to throw out there considering Disney's recent success with extra-ticketed events.
But I figure if I just keep riding it I'll eventually get to hear the verses to the song.
