So true that the 6 months before school is the hardest.
My elder DD went through a phase with the door too. She'd just wander off Luckily we have a quiet street, but with 2 police for parents you'd think we'd have that one covered lol. In the end I just had to make sure the screen was key locked and the key up high. A total PITA but she forgot about it after a bit.
I did also have to get DS's seat with the harness back out. He was not a fan of that at all. Thankfully though he saw the potential and I didn't need to install it back in the car (luckily as he's too big for it lol) When I get in the car and when I'm about to stop it I say "what's the rule with our seatbelts?", and generally he's pretty good now. Especially if it's followed up with a few "that's great you left your seatbelt done up until it was safe to take it off!!"s. They have to leave them buckled until I open their door.