Zahel is on Roshar because surviving off of Stormlight is less morally questionable than surviving off of Breaths.
Judging by the fact that he killed Shashara over it, I doubt he'd be willing to recreate the Nightblood experiment
While I do agree that a Shard's Intent has an effect on what is derived from its Investiture, I do not think Nightblood's strange nature is to be directly blamed upon Endowment's Intent, rather than on the vague nature of the Command used to Awaken him. Maybe Endowment's Intent did help make him able to take in other Investiture and that devolved into making him an Investiture black hole. Maybe Honor's Investiture would've been better & put in more checks & balances. But at the end, I think he was simply the failure of the people who created him and that failure could've been avoided with a little more forethought. They tried to recreate something they had seen, which arose naturally from a different magic system, they should've run some more tests (though it's understandable that they didn't given how many Breaths that would've taken)