1- nodes that use tor are connected to the internet through ISP that eventually block the port.
2- changing the port (I have to check if this is possible though) in your config and everyone's config will result to a dead network, got it ? think about it
1. It would not be a big issue. Nodes that can't connect will not connect and an alternative connection option can be produced.
2. A big issue might be if the ISPs blocked the port at their uplinks. This would form islands with different blockchains (by my understanding). However, a few modified clients could *bridge* the islands and the shorter fork would be rejected.
3. There is nothing that says that bitcoin must run on a specific port or even a single port. If ISPs started engaging in such behavior (and it would have to be some kind of coordinated effort), Tor, VPNs random ports and encrypted connections could be brought into play (this has all been done for torrents which face similar issues since ISPs are often hostile to it because 1) it eats bandwidth and 2) they often are in competition with the content that is being shared).
The only requirement is that a client be able to communicate with the mass of the network one way or another. And there's a lot of "anothers" available.
