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.
Any device involved in the conversation could have been compromised. In that case the entire message history would be intact.
Expiring messages don't completely solve the problem but it can drastically reduce the amount of information available.
So did the government come into possession of those messages by getting a warrant for the phones and discovering the messages post fact or is there some indication that the government was legally (or illegally) intercepting the messages over the wire?
I certainly know which one I think is likely.
"Get it, Jess. Do your fucking thing. This is what we fucking [unintellgible] up for. Everything we fucking trained for."
Sure, you have end to end encryption but that's hardly all of the threat vector.
People need to accept that it's far more than metadata, we have no privacy, and in a way we are all locked inside of these virtual jails that keep people locked in like drug addicts.
Good conspiracy though.