I think we can be pretty damn sure that the encryption is not backdoored since the Signal Protocol is the gold standard in encryption nowadays and thousands if not more highly skilled cryptographers without tied to the US govt looked at it thoroughly. Also Snowden calls Signal the best messenger on the grounds on him using it daily and still being alive so that's also a pretty good sign.
Also, do you have a source about them being mainly funded by the US govt? In their blog they talked about mainly being funded by small donors and a few initial loans from people who care about privacy.