You played it well, IMHO.
The rules are missleading about the player controlling no country in the variant without the investor card.
Here is how the rule should have been written :
After any nation takes its turn, all players may invest in that nation. Furthermore, players without any flags may invest in ANY nation (not just the nation that concluded its turn).
This problem has allready been tackled in another thread : here
(still waiting for the author confirmation)