Andrew, thanks for your efforts in fixing this issue as speedily as possible. However I'm sorry that I must disagree with your statement about the effects of the available to/from dates not being standard across the different activity modules.
In each case, the activity description/introduction is always displayed and the available to/from dates specify when students can participate in the activity. It has always been this way, and it seems everyone understands the functionality, as I've not come across anyone saying otherwise in the forums or tracker. Also, the effects of the available to/from dates are explained in the documentation for each activity module.
Thus, I would recommend that the assignment, data and lesson strings are left as they were i.e. the changes in cvs are reverted. Instead, I would recommend that a help popup is added to explain the new conditional activity settings and if necessary strings in condition.php are changed, rather than strings which have existed in a number of versions of Moodle.
A further reason for not changing well-established language strings is that many books and manuals have been written about Moodle and users become very confused if the Moodle interface is different from how it's described in the book or manual.
Regarding the new conditional activity settings, to avoid using the word 'available' again, the settings could be changed to 'Accessible from/to' with a help popup:
Accessible from/to dates determine when students can access the activity via a link on the course page.
The difference between accessible from/to dates and availability settings for the activity is that outside the set dates the latter allows students to view the activity description, whereas accessible from/to dates prevent access completely.