The idea with the Crown use is to find undiscovered magical potential in the list of properties for sale. Like a vein of magical material, or a long lost ancient buried temple, or dragon king eggs, or a Dragon's Nest. Something that Thomas would have no idea is there. That the people selling it would have no idea is there.
The "both" version with Thomas holding the mine property is actually perfect, I think. It explains how Molly got money for her "innovative process" - Thomas, who knew her through Harry, lent it to her. When she demonstrated that it worked, he established a factory near his small mine. It works.
Indeed. That's actually a perfect cover. The "official" timeline goes like this:
1) Thomas meets Molly through Harry. Molly, in her spare time, is trying to make diamonds in her family garage. Thomas floats her some money because he's rich and she's a nice kid.
2) Thomas goes on vacation to Africa. Something reminds him of Molly, and he buys a small mine, because he can. And wow, it works. He now becomes richer.
3) Some time after that, Molly comes to Thomas, and demonstrates that she actually worked out something innovative. Thomas capitalizes on it, and opens up a factory near his mine.