Honestly I don't know a great deal about osgood slaughters specifically. If your problem is as serious as you say it is, I agree with the advice given that you should see a professional in person rather than rely on internet advice.
Find a good sports physical therapist rather than a trainer. In my experience doctors tend to be over-conservative in their recommendations regarding weight training, and most trainers aren't likely to have much knowledge about specific knee problems.
Thing is you need someone to watch your movements in person. Particularly if you have one knee worse than the other. Remember that everything is connected, a problem at the knee can quickly turn into problems at the ankle, hip and lower back if it's causing you to lift unevenly.
Do you get pain when squatting to full depth even without weight? If so, it's probably a good idea to work on that before you start worrying about squatting and leg pressing with reduced ROM.
There's plenty of time to get big and strong but you only get one set of knees. I guess the main point I'm trying to make is to strive to get your knees as healthy as possible rather than just trying to work around the injury.
Oh and FWIW, most people I know have calves bigger than their arms.
Hope this helps. | 
