Stupidity: The Nostromo essentially is a detachable ship attached to a mining platform. After it returns and docks with the platform, much of the movie occurs in the platform section and not the Nostromo itself. The supposed reason they can't just blow up the ship is that the shuttle can't fit everyone. Why couldn't they just lure the alien into the platform, seal the ship, and then use the ship itself to escape? They could then blow up the platform to kill the alien.
Suggested correction: Dallas tries to flush the alien out of the air ducts and is presumably killed. Lambert suggests fleeing in the shuttle, but it cannot support 4 people. Ripley decides to continue with Dallas' plan, but before she does so, accesses Mother and finds out Ash was ordered to help bring the alien back alive. When confronting Ash with this, Ash attacks Ripley and is revealed as an android. The survivors then decide to go with Lambert's plan because now there are only 3 people. There is no error here.
Stupidity: When Jody traps Michael in his room, he does so by sticking a screwdriver between the the door and the jamb. This likely wouldn't accomplish much regardless, but it especially wouldn't work in this situation because the door opens into the room, so Michael could simply open the door and the screwdriver would fall on the ground.