In answer to your questions:
1. This is not the default functionality of the theme, so it would currently require customization on your part if you were looking to set it up as described. We have plans for a future release of Taskerr to include escrow payment functionality which will enable you to offer percentage based fees - however the release date for this is not yet confirmed.
2. You can find available widgets for the sidebar on the Taskerr theme admin demo
here which includes categories.
3. This would require customization on your part. Our themes are not encrypted, so you are free to make modifications if you wish. Thanks.