Crime scene in the chapel

Axioms

Regardless of whether they were alive or dead, the six definitely entered trough the door
Only one key to the chapel exists!
It is impossible to unlock the lock to the chapel with anything but the chapel's key!!
When the door to the chapel is locked, it prevents any and all methods of entry or exit!!
The six definitely entered trough 'this front door'!!
This morning Rosa definitely took an envelope out of Maria's handbag... and from that obtained the genuine key to the chapel.
The key to the chapel truly was the object inside the envelope I gave Maria.
The envelope I handed over to Maria and the one Rosa opened are the same thing!!

So far Beato has refused to repeat

"No one was able to touch Maria's handbag until Aunt Rosa took out the envelope"
"That envelope was completely under Maria's supervision until Aunt Rosa opened it"
But the final red truth in Alliance reveals Battler's theory to be just a red herring.

From the time Maria received her key to the instant Rosa unsealed the envelope the next day, the key passed through no one's hands!!
There were no doors with auto-locks other than Kinzo's study!
The six were already dead by the time they were discovered! All of their deaths were homicides! All six were genuine victims and did not take part in a mutual murder! There was no simultaneous murder!!
There was no one hiding in the chapel. Therefore, the shut-in murder you suggest does not work!
When the six were murdered in the chapel, the culprit was inside the chapel!

Transcript of the conversation in the Chapel the previous day

Corollaries

The chapel door was open all along

Rosa was persuaded to do what the culprits wanted, she acted her part and made sure that the crime scene would look like a closed room but it was never closed to begin with.

Leads:

The only solution to the red that we could find is to say the door was not locked when the key was sealed, and Rosa has to be lying for it to be true.
This way the culprit (Sayo or X Beatrice) was able to kill the victims in that room and then get out.
She is the only person we see examining the door.