Truth be told I personally do not know why it does nothing when you run that complete_migration script. Whenever I run it when upgrading from 1.7.5 it does it job right.
Have you looked at your cache/log.php in case there is anything new there?
On the other hand, may be there is something odd in your database that prevents the upgrade to go on? Or may be it actually runs but for some reason records are not being created on fo_processed_objects table. Have you tried logging in to your system afterwards? Are you able to see all of the tasks, notes, documents ,etc.?
As to why the version.php still says 1.7.5 I am not sure.. it should not if you delete the old version.php file and overwrite it with the new one which says 'return 2.1;' right? Are you deleting the other folders as suggested in the Wiki?
Last but not least, next release has already been improved and you will not have to go to 2.0.1 before 2.1 for the major upgrade. This was not supposed to work this way, but due to a bug that was left - among several others as you have noticed - there was no other way than this until the bug got fixed.. which it did now.. so to sum up, next version will allow going from 1.7.5 to next version. However, we are still working on how to improve the 'complete_migration' process as for people with a lot of data and not powerful servers this may lead to timeouts considering the fact it may take many hours, so for the time being it has to be run through the console..
Should anyone happen to help us improving this very process, please feel free to drop me a PM or an email.
Best regards,
Francisco