Got a WP7 i937 Focus S and tried with the latest 7.1.87xx aka WP7.8 today 12-17-14. Tried for about 2 hours to interop and then WP Tools root it. Tried method #1, then #2. #2 seemed to have the best chance of success as I got to the file manager and selected the HeathcliffInterop choice at the bottom of the file manager. I then back out to the start screen, try to deply WP& tools and still get the error 0x81031020 cannot be deployed error. Went thru the entire process 3-5 times same result. I know Heathcliff shared in his instructions that there are all sorts of reasons for the interop to take or not, just wondering if there aree any hints I am missing.
BTW...I went here http://www.wp7roottools.com/index.php/downloads
and downloaded Samsung G2 v3 and followed, well I think I did, the info in the instructions.txt file.