Original posts:
http://ramblingsofthegoldenwitch.tum...join-after-the
and
http://ramblingsofthegoldenwitch.tum...p-with-rudolph
It's the same question you asked here.
For example, we use Battler as the culprit child and Rudolph as the 'culprit of the first twilight who killed six people'.
After Rudolph killed the two people in Natsuhi's room, he left the room unlocked.
Battler told the servants (they stayed as a group in that time) to lock the door. He could told them something like this: Battler had a talk with Natsuhi and Krauss in that room and after he left, he noticed no 'lock click'. Worrying about this, the servant should lock the door for real using the master key but not 'distrubing' (the dead) couple. The trick is simnilar to the trick Natsuhi and Krauss were pulling on the siblings to prevent them to speak/meet with Kinzo.
Battler (or culprit x) could just order the servants to do that.
Notice the trick could be also used on the 1st twilight. It just require a servant who don't tell he/she lock the door because he/she didn't remember it or feared to be suspected.
I think Ryukishi had in mind that nobody than the culprit is responsible for the construction of the closed rooms. The rule 'non-culprit characters do not cooperate with the culprit' seems to prohibit it. Manga-Battler states to the rule: no secret agreements and accomplices, no covering up and giving alibis to the culprit. Also, in the blue truth he used Battler as a aid to let Rudolph in Natsuhi's room (they would not Rudolph in if he tried it ). Therefore the author kept the rule intact if the non-culprit character does not know the true intentions of the culprit.
In other words: The culprit can use innocent characters as aids if they think nothing is suspicous, that's the premise.