I was speaker at the Joomla day - Nice and presented the Joomla migration and upgrade.
I also be able to discuss with some people for the publishing of the fixe in the J3.4.2 and I have a good news for you.
After almost 1 year and a half, the issue present in J3.x concerning the processing of the error during the "Fix DB" should be released in the J3.4.2 and therefore, the upgrade could be done directly from J2.5.28 to J3.4.2.
In the current "french" powerpoint - page 34, I have given some "trouble shooting" solution. Perhaps that this will help you.
www.jms2win.com/en/documentation/migration-joomla