I saw you say that no child is born evil, which ofc is true, but no child is born ‘good’ either. We become ‘good’ based on our actions and our sense of good is largely shaped by our environmental upbringing.
The reason why Dragon was only able to save shanks, after their dying mother begged him to, is so at least shanks wouldn’t fall victim to the indoctrination of the Holy Knights, and trained to serve the WG. He was raised outside of that rigid environment, shamrock was not. I think it would be cheap to make him a right hand man, they’re being introduced as polar opposites for a reason, and the larger message being conveyed here by Oda (I presume) is that Shamrock COULD have been just like shanks, if he was not raised by his father to serve an imperialist regime.
Lastly, not sure what shamrock can even do if he was shanks right hand man, Ben beckman would be erasure and this sorta reveal would be redundant.