Sounds like a terrible day out to me. I do not know the spot you are talking about which makes it a bit hard to comment - I am going to assume its a Point break though.
Q1. Yes this bloke has probably snaked you. But were you at the top of the point or down it a bit so he could paddle out another 20m from you? Essentially he should have probably waited for you to get the first one. He might have seen you miss a few that you shouldn't have though and then decided to take the next one. Despite the fact that he did take the wave that you thought was yours it is your responsibility to make sure that you are not dropping in no matter what the situation is - not seeing someone is no excuse I reckon. If it was me I probably would have let it go the first time but said something when he paddled out. If he did it again I would just blatantly drop or paddle him out of position or push him too deep until he got the message that he can not repeatedly snake you.
Q2. once more hard to understand the situation with out witnessing it - but essentially the person paddling out should take it on the head and keep well out of your way. The reality though is that I hardly ever see anyone do this anymore. Infact when I do it most people are shocked I reckon!! I just usually take the line of least consequence - i.e. I assume the person paddling will aim for the shoulder and might not be able to duck dive so I prefer to potentially ruin the wave for myself by putting myself in a safer situation - making sure I avoid them at all costs - whether this is pulling back inside or heading super wide on the shoulder. Again I always assume the worst. I reckon you were dudded big time - but I can not conclude that having not seen the situation unfold. I do feel for ya though.
Q3. That guys sounds like a tool and should not be allowed in the water behaving like that. I am not a violent man but probably would like to see him get a slap (Hawaiian style) for that. Did he apologise & offer to pay for the fix? Was there any retribution ? How old was the fella? Did your lady annoy him previously?
Q4. If he had already done it once or twice - you need to mange the situation yrself - get on the front foot - either say something or at least be aware of what he is doing so you can avoid any potential problems / danger as they develop - if he has already done it 4 times there is a pretty high probability that he is going to do it the 5th time. So even though it might be wrong its not like it was a surprise to you as you had seen it before - so if you are not going to communicate to him that you would prefer he not do it and accept it then he will just keep doing it - so you need to be ready when he does it.
It sounds to me like you have done nothing wrong and unfortunately had 3 bad incidents in one short space of time. 3 incidents that well could have been avoided if things had gone your way. Its hard to conclude too strongly on your side as I do not now the other sides of the stories or the history but based on yr account I feel for ya. Personally I try not to surf crowded spots. Hate them. I am always on edge and assuming the worst - so I find it unenjoyable. I hope things improve for your next session