Keeper of the Lake's boss cutscene isn't anywhere near as long as I remembered it being, after warning my party that I was going to watch it when I was on a story replay. Maybe longer than the average "boss appears and roars" but not that bad. There's a long scene after the end of the dungeon and I think that blurred in memory.
I wanna say the last dungeon that has any serious cutscenes is the final dungeon of base HW
A way you can lower the chance of people engaging while you're watching cutscenes is to run dungeons first time as tank or healer. They're less likely to jump in, and then, if they do, they're liable to die.
What you described almost happened to my Let's Play alt. The only reason they didn't kill it, is because they didn't break its tail, and when I got out of the cutscene, it was at 20% with the entire arena covered in poison that it places with its tail heh.
I've since level DRK on this alt, and if I know a dungeon has a lengthy boss CS, I play it on tank to prevent missing out on showing the fight.
Unfortunately it does happen, if I see someone watching a cutscene I will wait with them regardless, this doesn't become as much of problem as you go through the game the expansions fix this by putting cutscenes at the start and the end, or they just do an extremely quick one to introduce the boss.