Yes, unfortunately. Car seats and booster seats are hard items and they don't have joints to move bits out of the way. People are squishy and have moving parts. The adults won't be comfortable if they are thicker, but they can squish in - often, quite literally.

The solution to this would realistically be to have built-in booster seats or a different style of seat belt that would better fit children and eliminate the need for booster seats for the older children.

Yes. If you have 3 kids and 2 are in the booster seat category, which some agencies recommend until they are 4'9" (145 cm), which some kids may not reach until 12 or older. You cannot fit 3 large booster seats in a Golf, or at least not comfortably.

I personally doubt the necessity of booster seats for older children at all but that's another topic all together.