Finally cracked what is going on with the checksum. The CRC algo is running on an 8 byte value, but our data is 7 long. The low byte is being padded with 0x00 due to the buffer being initialized to 0x0000000000000000 and the packet shifted in. CRC byte is added to the end of the packet, there appears to be a zero stop bit or some other inter-packet padding that rtl_433 was interpreting as data. D'oh!

Yeah, this is my first time reverse engineering an RF protocol...