If our software can submit a valid share to a pool, it can also find a valid block and thats all the proof you need.
But can is too little here, it has to.
You can ask slush pool and Genesis mining about that stuff
So here my question again:
Why is the backend code not published under an open source license?
I mean its BTC, small nice community project...
Do not trust, verify!
I like the product. Bring mining back to small people, decentralized, quiet etc. Would be happy to support the project and preorder.
But for sure I will not buy a closed source miner, which has never shown that it can find blocks.
Especially not if it is to be used for solomining. (Whether this is right for the target group "are first time miners" is another question.)
I really don't see any understandable reason not to publish the code.