1. Seems hugely unlikely, for reasons already repeatedly mentioned. I suppose there's a chance we're the first, although unless there's evidence that our solar system is one of the earliest, I don't see much point in entertaining that line of reasoning. There are plenty of other reasons for why we wouldn't have heard from aliens, even if they exist, which I'll mention as I answer the other parts.
2. Human radio signals have only traversed less than 100 light years into space. This means they have only reached a relatively small number of stars. Aside from speculative technologies based on rather poorly understood physics, there's no feasible way for an alien civilization not within that range to have heard from us. If aliens with such technology exist, it may be that they are aware of many, many other civilizations like us and simply don't care. Otherwise, they might target us for conquest if Earth proves sustainable to them. Any truly intelligent race would probably stay the fuck away, considering that if we still kill each other over trivial differences among ourselves, we'd probably have no qualms killing people of a different species.
Quantum entanglement seems to violate relativity (or the collapse does, at any rate), and if we manage to manipulate space (in ways that it theoretically could be manipulated, according to our current understanding) there are loopholes around it. I don't expect interstellar travel to happen this century or the next, but the prospect of what technology could look like a thousand years from now is mind boggling. Thus, I don't necessarily rule out the possibility that some alien civilization can travel the stars. It doesn't mean that we would notice them, or they us.
3. Finding other life forms before we find them (yes read the OP) is a far more grievous violation of relativity than interstellar travel, but I suppose NASA would try to contact them by radio (though it would take a while), unless the government didn't want them to or something. I don't think there are any laws about it though, so if they don't want to be told "no" they don't have to ask... At any rate, what SETI has done so far in search of alien life is equivalent to dipping a cup in the ocean in search of fish.
4. If we find them, it will just blow minds, but nothing else. If they come to us, the results could be anything from the end of mankind to an era of prosperity through trade and learning. I doubt we will catch their viruses (or visa versa) since even if they have DNA and use it to create proteins from amino acids, they would likely use different bases, or at least "read" codons differently, meaning that our viruses would be unable to effectively copy themselves in the other species' cells. I think it's likely that at least some life out there uses similar chemistry to ours, even if some is also completely different.
5. We would go from having one example of life to having two. That would greatly increase our understanding of how life forms, as we'd have basis for comparison. Perspectives might shift in response to their scientific or philosophical understanding, as well. While we may be extremely different, I find it highly implausible that two sentient species trying to communicate with each other would be completely incapable of doing so, especially with the aid of computers. I think most religions would struggle to explain the existence of other forms of life, although fundamentalists are so good at ignoring evidence that I wouldn't put it past them to rationalize away how it still works with [holy book].
