The rules to deprecate and remove capabilities from Adobe Experience Manager are as follows:
In the release notes or for APIs in code, Adobe will mark a capability as deprecated in a given release. Then customers have at least one release cycle time to switch their implementation to the replacement. It says 'at least' as a capability might be removed in the following release the deprecation announcement was made - or any release after the following.
Classic UI has not been announced as deprecated in 6.1 and it won't be the case either with the next release in 2016.
Yes, since the release of the Touch-optimized UI Update Pack for AEM 6.1, it is possible to include several richtext components in a dialog definition.
In 6.0 , it is still a known limitation. The solution in this version is to use an Hybrid Editor as a workaround.
Refer to the Text & Image example in 6.1 OOTB, described as well in this presentation (slide 47-48).
See http://docs.adobe.com/docs/en/aem/6-0/develop/extending/customizing-page-authoring-touch.html#Add New Inplace Editor for more information.
No, the Image Editor component is still only usable in inline editing.
In 6.1, the Touch-optimized Update Pack enhanced the inline editors, and now offers an easy way to configure multiple inline editors for the same component.
The procedure is fully described in this documentation page.
In 6.0, you would have to use the same workaround as for the RichText editor, described in the FAQ just above.
Validation in Granite UI is achieved by using the jQuery Validator plugin.
This replaces the former vtype validation used in ExtJs widgets in the Classic UI.
An AEM Documentation page is under preparation about this.