- Is it, or wil it soon be possible, to post media content on the walls of groups?

Yes, once CB Activity integrates with GJ this will be possible using the Links feature. Otherwise at this time you can share media by simply adding the URL to your post.

- I have an extension that needs to post content on group and friends walls. Where can I find documentation on how to develop a plugin for this task?

There's no documentation for such a thing. We've very little API documentation. Typically you can just directly insert your new content into the database.

- I need some tool to perform recurring administrative tasks, Auto Action looks like a suitable candidate. Is it possible to schedule these action using CRON?

Yes, each auto action is given a direct URL to execute it that you can use with CRON. Additionally there is an Internal Users (this will loop your auto action through every user, be careful with this one) and Internal General trigger that you can assign to auto actions then execute the respective internal users or internal general URL from CRON.