As far as i know, the ringwraiths are meant to reclaim the one ring, so if mormarath found the ring and wore it, i dont think the witch king will find him as great problem since all the knowledge of mormarath in magic is nothing compared to him, and mormarath cant trigger and use the full power of the ring, the ring will destroy him and be in the witch-king's favor in battle since he wants to come back to his master.
I would class Gandalf and Aragon as the most important heroes in Gonder, however, when Boromir takes the ring, Gandalf is still as important as before and Continue to aid the kindom of gondor, Aragon didnt sit on the thron of gondor untill he was as old as my grand father, most of the time he is just a ranger, Boromir is the son of denethor who are allowed to rule gondor when thier is no king, so even when boromir takes the ring and everything focused around him, the other important heroes of gondor didnt loose anything of thier place in the faction and the faction characterization is not effected,thats why it fits perfectly for boromir to be a ring hero unlike how the witch-king will be replaced in angmar.
Now you are saying the forces of angmar will serve mormorath out of fear and because he is currently the strongest, yet the witch-king will come and slay him, how did the witch-king kill someone stronger than him? reinforcment from mordor? that is not possible at all, you know how far mordor is from angmar? not to mentions all the good factions that the witch-king will face on the way, there is no way that mordor troops will make it to angmar, and as far as i know at angmar time, mordor was not as strong as at the battle against gondor, and i dont think its a good idea to see mordor troops in angmar faction game play-wise.
The wizards cult of Angmar follow Gulzar (witch-king's best student) who is very loyal and grateful to the witch-king, for him the witch-king is not just a master but a teacher as well, so the wizards wont betray the lord of all witchers.
If the troops didnt obey mormarath then you would have mormarath and his men Vs the witch-king and the orcs hillmen wizards wolfs barrow wights trolls, the witch-king is sauron's strongest servant and he gave all his forces the ultimate promises i dont think its a good idea to leave such a good leader and follow mormarath out of fear, since he will treat them miserably, and will throw them to battle and not care about them since he only care about his men and he look down to other races, and he already cant offer more than the witch-king to them, there is no great trade off that could convince the armies to betray the witch-king.
"his own folk quail at him, and they would slay themselves at his bidding." - J.R.R. Tolkien
Thats another point why I am against mormarath as a second ring hero is because I realy like the theme of sacrifice in the faction and how the forces of the witch-king ready to pay their own lives to archive victory, and in my opinion it doesnt fit to have this sacrifice theme and a rebellion in the same faction.