1. Malicious exit nodes can correlate your BT streams to your Tor web browsing, and learn your real IP.
How exactly can they do this? Why would your web browsing have any correlation to your BT streams?
Sorry, should have been more clear. Because Tor conserves circuits for reasons of efficiency, it is possible for an exit node to build a profile of the activity of a circuit by inspecting the data leaving that circuit. If you are browsing via Tor while running a BT session, the data from the two sessions can be sent over the same circuit. The exit node can learn your IP from the BT stream (BT client tells tracker what unique random port it's listening on, exit node sees connection to tracker at unique port number, faulty BT client sends IP to tracker) and correlate that to the web traffic on the same circuit. They now know what you're browsing, and what your IP is. Anonymity broken. It's explained in the post I linked to.
Like you say, if your BT client doesn't know its real IP (NAT etc), then you're OK. It's a question of all the lemons lining up.
Assuming you don't want to actually download anything. What is actually available on I2P? How does its library compare with any of the trackers on the internet at large? The reason people use tor isn't because it is more secure, but because it lets you browse the internet that you already use.
Chicken and egg problem. If more people used I2P for their filesharing, then there would be more files available over I2P. A few benevolent individuals are seeding more and more. But yes, I understand that people prefer to proxy their normal things over Tor rather than switch entirely to new networks. It's just a shame that in doing so they hurt the Tor network.
There was a suggestion a while ago that someone ought to make a bitTORrent client which ran a Tor relay on every BT peer. This would solve the bandwidth problem. Don't know if any work was ever done on it.