Hey there!
I must be something like Cursed

So yes, I tried to double spend when i got this Missing inputs error... Because I did a test transaction from CoPay, which in fact spent the input i was using lol
soo.. trying to use a different input, now i'm a little stuck getting my parameters back together to produce a valid transaction payload.
I'm now using the input
vout=1 in https://api.smartbit.com.au/v1/blockchain/tx/517a0ad4bf4cc423ce578f043a13e98d405902c08b1bfcac92b199ba3fd2cc39. Does this change something to my process ?
No need for the OP_EQUAL
I removed it and put it back in during debug.. Thanks for re-mentioning

I will have a look also at importing keys in omnicore directly. Thank you for the details.