It says on the manual that the Director's share certificate is reserved for the owner of the associated coal railway.
However, it says that the 10% shares are available when the Coal Railway is formed. Isn't it possible for a player to buy a lot of stock of that company and end up having more stock than the owner of the Coal Railway? Does this major stock-owner become the director, when the company begins? If so, is the trade of certificates (two 10% for the director's single 20%) automatic?
I really have to say that my guess is as good as yours. I'd say your reasoning is correct. It seems the most logical way to go.
It says also that the company has to become operation when 50% of its shares have been acquired and the Director's certificate is in the hands of a player... So, the company will only form after the (optional or mandatory) exchange of the coal railway, right?
I'd say yes. Again, this seems to be what the rules imply.