How to control editing access when node ownership keeps changing?
See my recent answer here: https://drupal.stackexchange.com/a/55144/7153

But basically, you can use

nodeaccess_userreference

or

nodeaccess_nodereference

to acheive that access control functionality there.

Basically you could deny author access and control edit access with one multi-value reference field instead.

More info, including screenshots at first link.