Thank you for your questions. Let me clarify:
1) Only Prometheus RadControls are based on MS AJAX framework.
2) "The old " RadAjax (currently official v1.8) is a separate framework. There are some limitations in the new Prometheus RadAjax just because it is based on MS AJAX, which the old one does not suffer from. You can review this help article
for more information.
3) New features are implemented mostly in Prometheus package as soon as the respective control is migrated there. However, we will continue fixing available bugs in the old RadControls package.
4) Both "old" RadControls and Prometheus can coexist in a single project. However, note that we recommend using single AJAX framework in order to avoid compatibility issues.
5) You can open support ticket as well as start forum threads. I believe, you can already find the answer to your questions (or at least in 99% of the cases) in the forums or in documentation.
I believe you will find more useful information about Prometheus in the FAQ page here
All the best,
the Telerik team