Just replace the suspect valves with Marlon valves and hose adapters.
As you said, the marlon valves will correct the thread mismatch you have now.

It is quite likely the existing valves are not marine (steel nut, galv. handle ?).
But there is no such thing as a non marine thru-hull; Could be bad quality, but it is marine.
Sure the flanged base seacocks or adapters are better, but is this in an area where the valve might get bumped or stood on ?

Cheers,
John.