Not sure about the wp-cron problem (doesn’t sound like it’s related to this change?).
The activity uses any “post” activity, i.e. whenever the user creates a new blog post or page, he/she jumps to the top of the list. Comments are not considered at the moment and it doesn’t do any weighting either of course.
I could add recent comments into the mix but it would still just be a sorted by “recentness”; so whoeever posted the last comment/post would appear on top. For a “better” activity sort we’d need some kind of activity points system which factors in the time of a post/comment, activity type (post, comment, or something else) and possibly other factors like the custom weights you wanted; I can see how that would probably be quite useful but it’s not really in the scope for this plugin.