I agree with the PTO engagement. Just yank on the handle to engage. There is no mesh at all it's simmilar to shifting your car into 1st gear without a clutch. This is why it's recommended to be done at idle.
As far as your 50" deck slipping I would guess its the slip clutch slipping, or possibly your belts on the deck are loose. I have a similar issue with my snow blower. Even after adjustment the clutch begins to slip in about 4" of snow. I think it's just slipped so many times its just worn and extra slippery. I'm not sure if that is really the case, but I'll be I disassembling it next week or so and reface the metal surfaces on my lathe and replace the fiber washer.
Alex