I would assume the most likely explanation is an informant was included in the Signal conversations and forwarded the contents along. I wonder if we will get an official statement from Signal.
When you have multiple sources receiving (ie they are the intended recipients) decrypted messages that corroborate each other, the encryption aspect probably becomes all but moot.
Not sure it's more informants, but they do seem to use more than one
Unless you just mean completely invent evidence on a controlled device in which case sure just edit the local message database like you would anything.
which means that once a message is received you cannot re-authenticate it, so evidence on any device is no better than with unencrypted messaging. This is in contrast with classic public key authentication like PGP which provides further evidence that the message is legit and was indeed sent by who it says.