First, I think you have the master and slave memory addresses backwards? If you wish to write the data in V10000 and V10001 then you would use LDA O10000 (the master source data address). Then put V12004 in the WX instruction (the slave destination address).
Also, if you want to write V10000 (2 byte register) and V10001 (another 2 byte register), then this is 4 bytes. Change your LD K2 to LD K4 (it's number of bytes, not number of registers). This will actually write to the camera's hex modbus addresses 1404 and 1405.
Your questions about the addressing are quite relevant. In Modbus, the user addresses are decimal, starting at 1. However, at the protocol level, the addresses are zero-based, so the client/master protocol software will subtract 1 from the "user" address, giving the "protocol" address (the bits in the bit stream). Therefore, if the Basic code that simulates the PROTOCOL (i.e. it's a protocol address, not a user address), says to use HEX 1404, then I am guessing that your calculation is correct (I think ECOM100s need protocol addresses, not user addresses???).
The best way to tell is to see if you get your desired results. It's bad enough that the Modbus wire protocol address is off-by-one than the Modbus user address, then throw in the ADC element octal mapping you have to use, it gets a little crazy real quick.
Someday, we'd like to have a Modbus RX/WX IBox for the ECOM100!