Typically for the returns you'll want to use absolute non-SEF. Relative SEF I've no idea how well it'll work as throughout Joomla and our plugins they use and expect non-SEF absolute. It's safe to exclude the domain and simply begin with index.php though.
Was able to confirm the base64 decode isn't working right. Adding a bug ticket and fixing for next nightly.
forge.joomlapolis.com/issues/5929
It is unlikely AND/OR usages will be implemented anytime soon for conditions. I can't even imagine how the UX would function to do that in a way that wouldn't massively confuse everyone. As is it's simple and easy to use. The AND/OR cases in most usages can easily be handled with a REGEXP operator (e.g. [cb_myfield] Is REGEXP /Value1|Value2/).
With that said I do have some ideas on how I'm going to possibly handle AND/OR though in a future major release and made a ticket to note them below.
forge.joomlapolis.com/issues/5930