All these things can be done by a custom module for CubePoints.
– donations as an alternative to “liking” a post
Would you “donate” a specific amount of points or as with the donations module, any number of points?
– Donate to author, directly from the comment area of a post
Your theme’s comments.php file would be probably the place to insert this. There are a lot of different places this could be included, the question is where.
– have accumulated points public on the post
This really depends on where you want to display this information. You could write your own custom function in your themes functions.php file where you query for a particular posts “donated” points. The post would have it’s authors ID under post_author and along with the post_id a simple SQL query would sort this out.
– top ranked post widget
Have a look in the WordPress Codex for how to make your own widgets. There are tons of nice tutorials out there.