I don't think a Marlon thru-hull is ever a good idea UNLESS it is fully protected by a bolted down flanged base seacock.

Now that you have some confidence in your thru-hulls why don't you leave the rest of them in place and just replace the valves ?
It would be a two person job: one on the outside to keep the thru-hull from rotating while the other person removes the old valve and installs the new.
The outside person should apply some pressure to oppose the unscrewing of the old valve then switch to opposing the tightening of the new valve.
If the thru-hull moves during this operation then either remove and re-bed it (if it can be removed in one piece) or replace it.
The thru-hulls probably won't move even without outside help.

Cheers,
JM.