I don't honestly know why having the host club next door is such a disaster. It will only attract some girls who are still interested in those disinherited guys, who never pay attention to any other girl than Marie. They are of no consequence to Leon's cafe. His biggest obstacle is his own infamy. However, that being said, how could it be that everybody would have lost money in the betting or wanted to be friends with the princes and whatnot who form Marie's harem? There should be people who either didn't bet or bet only an insignificant amount (and maybe a few who went all out and placed their money on the underdog). Furthermore, there should be people who actually appreciate a capable person who made his own fortune by adventuring like the original founders of the country. Of course they are still all young students and many would be sheeples, but nonetheless there would always be some with brains of their own. When those go in to visit, even some indecisive ones would follow.
What comes to the queen, somehow I suspect she's the last person whose attention Leon wanted to gain.