The founder is fortunately technically versed enough that he should be capable of an audit of the work conducted by his employees. However, I think he's clueless to what's going on at the moment. He's an awesome guy and his heart is in the right place, but he's much more sales-oriented and concerned with bringing in business than he is with auditing the performance of his employees. Unfortunately. I believe he is indeed too busy with with other things to see this and I think he thinks everything is going well in terms of operations (despite my vague alert). That said, business was booming several months ago and he was not concerned at all about money then. These past couple months have been poor though.
It could be that the founder keeps himself officially "unaware" on purpose. This way he can shift the blame to others if it becomes public, and still make a profit from defrauding clients.
If he is truly unaware, you will still be up against two senior employees who will deny the accusations - and you may end up looking like the "crazy disruptive new guy".
I would suggest gathering as much documentation as you can on the excessive billing. Then approaching him without directly accusing the senior employees, and pitching it as a way to improve the business. "I have figured out why the clients left, we made mistakes on their bills!" instead of "Your senior people are defrauding clients".
If the founder seems to be unconcerned about the "mistakes", start looking for a new job.
But give him another chance. Address the issues you noted, and see if he cares. If you think he's a good person, you probably can explain the importance of keeping good relations and making sure that customers are satisfied (and if he disagrees, you probably should leave)