Well there can also be confusion about naming conventions because Jaime Wolf and his brother both had the last name Wolf.
But the unit dossiers for Dark Ages have quite a few warriors whose last names are Nova Cat that also happen to be trueborn. For whatever reason its becoming more common to call them by that though. Maybe because they're so deeply entranced in the Inner Sphere now.
Jamie and Joshua got a special dispensation for their names - in effect the Wolf at the end of their name is a Bloodname by order of the ilKhan at the time IIRC purely so they would have enough authority to hold onto their position over the time away from the Clans. If they had returned it would have been interesting to see how that was stepped around... carefully I think
The Wolf's Dragoons adopted this policy with their "Honour names" being carried on through the generations. But there are few "Wolf" because the Dragoons hold them apart for Jamie and Joshua in memory
If you look you will see many Wolves named Wolf as well, but you will see few in other Clans, this is mainly because the Wolves and Nova Cats are two of the more liberal Clans and allow warriors to show their skill bloodnamed or not unlike the Falcons which see the Bloodname as the ultimate and most important symbol.
Phelan was renamed Wolf as well. And Elson Nova Cat was a freeborn Elemental taken at the battle of Luthien. He refused to accept the name Wolf, iirc. So I assume freeborns indeed serve in all the Clans present, and often have important roles.
Phelan was captured by Clan Wolf so he became Phelan Wolf. Elson as you mentioned held more loyalty to the Nova Cats than the Dragoons so he kept his Clan's name I think purely to spite the Dragoons.
IIRC all unbloodnamed Warriors, true or free, whatever their Clan are named after their Clan as it is the only way to distinguish where they are from. The Clans themselves don't really give a crap if they have a Clan name or not because it's not a Bloodname and so not important, or a true surname.
Warriors freeborn or true adopted into their Clan should lose their surname (if Inner Sphere freeborn - like Phelan), adopt their new Clan's name or retain their Bloodname as the Bloodname stands above any bond or rank.