Of course it's just an academic question. I don't have an aunt named Millie. And I'm not planning to name a rose after anyone who is famous.
I don't think it is possible to copyright a name any more than it is possible to copyright the title of a work. I also don't think that famous people even today actually register their names as trademarks - not even the ones with made up names like Sting. My understanding is that if they did, in order for the trademark to be completely enforceable, they'd have to reproduce the little TM symbol next to their name everywhere it is used in trade. I cannot recall ever seeing that done.
But if we were to assume Picasso's name had been trademarked, I expect that McGredy could have simply ignored the fact since the products they were selling - namely "artwork by a famous personality" and "roses" - would not normally be confused with each other. Under trademark law I could sell rocket engines using the trade name "Kleenex" with impunity. But probably not linen table napkins.
