Welcome to the forum! Sorry, I'm not sure I can answer your questions, but that won't stop me from trying.
Guess on the float is that it is either binding (so it only seals the fuel off when you pull it hard enough) or possibly it has a hole? Either of these would probably make you run rich though. So when you say leaks where the air filter attaches, what is the "leak" like? Fuel spray back into the air box? Liquid coming back somewhere? If the former, I'm surprised you don't get back fires that blow the air filter apart. If the latter, maybe you have a crack that the pressurized fuel is enough to breach, but not your mouth? All just speculation though.
On the fuel pump, I would think the plastic would work fine. Again just a guess, but I can't imagine it working in some applications but not others (as long as it fits your pump).
Best of luck and hopefully you get somebody with real knowledge on the subject to set me straight soon.
- Steve in CO