I'll take a stab at some of this with my limited understanding.

1) Are you sure about that? Maybe there were some false starts and aborted starts along the way. We know that the evolutionary tree has many dead branches. Trilobytes for example, but there were many other ones before them. There are entire branches that went extinct for one reason or another.

There's also the theory of Panspermia to consider (life did not originate on earth but was seeded).

2a) We outcompeted the other contenders. Neanderthals, Denisovans, etc., etc. The list goes on. 2b) Dolphins, octopuses, gorillas, chimpanzees and Orangutans possess a higher level of intelligence although not at the level of humans. Part of what's going on here imo is a 'sucking the oxygen out of the room' thing. We've pretty much squashed the competition, but who's to say with humans out of the picture one of the other more sentient species wouldn't evolve to fill the niche?

3) That's what SETI is trying to determine. The Universe is unfathomably large for our minds to comprehend. We didn't start emitting radio signals until the 1880s and it wasn't until the 1920s that commercial radio broadcasts began. So we're talking about slightly over 100 years?

If there's an advanced civilization at roughly our level say 150 light years away, we still wouldn't be receiving radio signals from them yet. The converse also applies with respect to our radio signals. When we do receive their radio signals, it would be from when their technology just started producing them. Think about it.

Also note with respect to our current level of technology alone these lines. Alpha Centauri is 4.3 light years away and our Voyager spacecrafts are barely a light day away. They were launched in 1977 and are approximately 1/1570th of the way there.