A. The Seder can be a 3-hour experience, sometimes even longer, depending on how its led and run. Some people love it and appreciate it, others maybe not as much. The Seder-within-a-Seder concept allows students who may not be able or interested to sit through the whole thing to still have some of the Seder experience.
If the regular Seder starts at 8 or 8:30pm, we’re near the conclusion of Maggid and up to the 2nd cup around 9:30pm. So those who come then get to drink a cup or two (maybe four?) , tell a little of the story of the Exodus, eat the Matzah, Maror and Korech sandwich, probably get most of the holiday meal, and maybe even get to open the door for Elijah (but that’s probably pushing it).
The Seder-within-a-Seder may not give you the full flavor. You’ll miss the Four Questions and most of the Haggadah readings, but you still get the key essential Mitzvot of the Seder night, especially the Matzah and speaking of the Exodus. Remember its the Mitzvot of the Seder that matter most!
And of course, no one is keeping those reserved for Seder-within-a-Seder to stay on for the rest of the Seder!