Have you tried deleting the validation error and revalidating? I’ve tried creating both a <Picker> and <Multipicker> without a caption, but I wasn’t able to get that validation error. If you’re willing to post your metadata model, we can try to investigate further.
The metadata loader does not recognize your control as a Multipicker. Can you try to associate the layout with the class that is the container of the BatchPrintActionCommandSignatory, and set the “items” property to the collection of signatories (e.g. items=”(@ signatories)”)
Please ignore the second error related to the TempFormRef. The metadata loader validates both the layouts (forms in pre 9.x) and the dialogs/portlers (screens in pre 9.x) separately and generates duplicate validation errors. The TempFormRef error should not be generated, it’s platform bug.