Are you saying when the index is in use "the same" query returns results ?

Multi Resource SupportActivating Multi Resource Support for your workflow model means that a single workflow instance will be started when you select multiple resources. It should be activated for your workflow model.

I would suggest you to check ths following doc where you can see how to create a workflow, invoke it, and monitor it.Adobe Experience Manager Help | Using Adobe Experience Manager Workflows

In Author mode, anonymous access is disabled for most part.You can enable anonymous user in Apache sling authentication service.

Check the relevant document to ensure you are not missing anything:Target Integration with Experience Fragments

You may want to check this:Obtaining Page Information in JSON Format

I have done this update and did not see any issues.try invalidating cache and rebuild libraries:http://host:port/libs/granite/ui/content/dumplibs.rebuild.html

You may not have a "search" requirements yourself. When we mentioned "search" it does not imply a search feature or something like that. Oftentimes AEM needs to fetch properties/nodes to assemble and render pages and for doing this a query is built and executed behind the scene . We are referring to...

On top of what Leo mentioned you should actively check the logs to see if there has been any traversal query and if there is such query just create a relevant index. You can use Oak index definition generator for this purpose.Oak Utilities : Index Definition Generator 

In addition to what Vishu mentioned also its good to check the following document:Post Upgrade Checks and Troubleshooting