I'm happy to chase down possible Transmission bugs, but so far I don't yet see how this is a Transmission bug. None of your crash reports even mention Transmission, so the only evidence in that corner is your statement that, to you, it "feels" like a memory leak in Transmission is corrupting the kernel.
What it really sounds like to me is that there's an issue in your networking configuration somewhere which is being triggered by Transmission. The clues that make me think this are:
- You say the crash happens after you start Transmission
- You say the crash is more likely when there are more torrents
- Transmission uses network resources
- Transmission can use more network resources when there are more torrents
- The kernel panic mentions the AirPort in its trace
- Your other mac doesn't crash on Transmission at all