ISSUE upgrading CiviCRM 4.6 to 4.7 (or 5.1) in Wordpress (comes from a Drupal multisite)
we were able to update CiviCRM from ver. 4.6.18 to version 5.2.1! I will tell you below how we solved the problem:

a) we realized that we had 3 extensions (CiviVolunteer + Angular + CiviRules) that we did not use. CiviRules, in particular, needed updates (a Yellow Warning said us that it was necessary to update the DB, but the relative link did not work);

b) then we unistalled all the extensions and canceled the relative permissions in Wordpress Access Controll.

c) we cleaned up and verified that no traces were left.

The upgrade was successful.

It must be said that we had installed the three extensions for testing purposes, but if they were in production ... :(