I ran into the following issue recently. I installed both tortoisesvn 1.8.2 and tortoisehg 2.9.2 (in that order!), and was having trouble pushing some changes to a hg-repo over ssh.
In the debug output it kept saying "no suitable response from remote hg" (or something like that, doesn't matter). It turns out tortoisehg was trying to use the tortoiseplink.exe from tortoisesvn, and not the tortoiseplink.exe in the tortoisehg distribution, because the tortoisesvn version was first in my PATH variable. I had to remove the tortoisesvn entry in my PATH to make it work again.
Is there any way you could make tortoisehg not look in PATH for tortoiseplink.exe but instead use the installation folder to call the correct tortoiseplink.exe ?
Best regards, wvc