An interesting couple of questions Baylor.
Napoleon did have camps at the channel for his army...but at the time of Trafalgar he had more or less abandoned plans for an invasion anyway, and he was marching full pelt to the East and for fortune and glory at Austerlitz.
I suppose the question is that if the British had been defeated how easy would it have been for the French to become the dominant naval power. I'm sure Britain would, like the Romans who never quit, would have re-built and tried hard to come back with another naval attempt. They didn't know it at the time but even if Trafalgar had been a win for the French, they were not really under threat of an immediate invasion. So they had some breathing space.
The French under Louis XVI had tried to wrest away British superiority from the seas, but that damn near bankrupted the state - something that I'm sure Napoleon was aware of. Also I don't think he ever showed any aptitude for navel warfare. So I think he always thought in terms of land operations and avoided the seas. I think it would have required a spectacular defeat of the British for Napoleon to even consider looking at any invasion or sea plans.
As for not invading Russia, my sense is that he'd have found practically any excuse to invade, just to have a victory like Austerlitz again. It was probably too late to rein back the megalomania and ego by that point. Military minds come up with military solutions - could Napoleon have 'retired' said "that's enough" and run the whole of Europe?