MB Builder *Required* validation option?

Hi, yes, that option is on the plan. We've just updated the UI, and now we'll scan all the settings again and added those options to the plugin. Please give us some time to update.

It probably will take some days. If that's urgent for you, you can use Custom Attributes to add an attribute required with value true to the field. That will make the field required.

Regarding conditions, yes, you should use the Builder to build that. It supports all conditions. If you need to do some extra check (like conditions outside meta boxes), then you need to tweak the code a little bit.