Warning: I am not an English teacher. In reality, whichever you decide to use would be interpreted the same way based on context. But, what I would do is: "If I had not bought that last ticket, I would be watching the game from outside the stadium now. " I would use "had not bought" because it communicates that you DID buy it, but you could have NOT bought it. You could use "didn't buy" whether or not you have bought the ticket already. Consider a present tense example: "I don't want to buy it, but if I didn't buy it my friend would be angry". You're kind of imagining a possible future where you did not buy it. I would use "I would be watching" because "be watching" is progressive and it communicates an alternate to what you are currently doing right now. Whereas "I would have been watching" could be used when you're talking about something that could have happened in the past. Meaning, you may not be at the game right now. You could be talking about the game later. However, the fact that you put "now" at the end of the sentence clears up any ambiguity regarding when this event would be happening and using either would be interpreted the same way.

Good question. In fact, what you need here is a mixed conditional: 'If I hadn't bought that last ticket, I would be watching the game from outside the stadium now.' The 'if' clause is a third conditional, because it's a hypothetical situation in past time. The second half of the sentence is a second conditional, because it's an unreal present.

"If I didn't buy that last ticket, I would have been watching the game from outside of the stadium right now!" Reasoning: "Would have been" explains the logic of an event. If one thing didn't happen, another thing would have happened. In your example, if you did not buy the last ticket, you would have been outside. I hope that helps a little bit! :)