So im not sure if by growing the scene you mean attracting pub players to sign up for a competitive tournament or just trying to get lower div people more involved in the scene.
We hosted a german TF2 cup twice now ( about 72 people played ) and it usually got the german scene quite active for the next 2 weeks or so. It was a pug like cup where 12 captains could draft their team. In the first cup there werent any limits other than the draft order. In the second cup we copied the frenchies by assigning values to the players and setting a budget for the captains.
Obviously youve got a way bigger playerbase in NA (no language barrier) so you could have a cup like this in a bigger format. Its a lovely way to get the lower div people in contact with the higher div ones. Im sure a lot of the new players learnt quite a lot in our cups and some even stayed around in the pug community.
About the casting situation: We just had people from the community doing something we called "community cast". Basically 2-3 people chilling around talking a bit about the game, hyping up plays and whatnot. Depending on what you are trying to achieve this chill type of a cast can also be quite entertaining.
Our prize pool was completely funded by donations cause nobobdy in EU is really willing to pay to enter a cup. Ive heard its different in NA though.
I love what you are trying to do. Good luck!