As per doc the approach is [1].  I doubt doc is valid & upto date.  Please file support ticket to track & get official statement from engineering team.[1]   https://docs.adobe.com/docs/en/aem/6-1/administer/integration/cq-as-portal.html

enable debug log Or use http://adobe-consulting-services.github.io/acs-aem-tools/explain-query.html

Hmmm..  Sorry missed the description.   From Explain query output your index not even listed that means your custom nt:base rule would over shadow the default nt:base rule.   Looking into your query & based on path "/content/demo-spark/jcr:content/bgimage"  I am assuming "demo-spark" is cq:Page. So ...

https://docs.adobe.com/docs/en/aem/6-1/administer/communities/srp/msrp.html

As temporary workaround add [1] into in the <a> tag. Later fix the code to use externalize api. [1]  x-cq-linkchecker="skip"

Sounds like you havewrong rewrite at /etc/map/http Ornot configured linkchecker to treat www.domain.com as internal. OrThe component that is generating url is not using resolver.If all are configured correctly file a support ticket to validate your settings.

Adding XMP Writeback step to the DAM Update Asset workflow  is correct & I would say temporary workaround.   Can you please file a enhancement support ticket because this should happen oob.

surely 1 is not correct.   It is some what like 2 but I do not have exact syntax for lucene since I implemented this feature for AEM-solr integration.  I need to do some research for lucene if you are unsuccessfully please file a support ticket so that we can get answer for you.

With AEM6 it does not work. The alternative is configure use in suggest [1]  & then use native query.  OOB sample is planned for next release.[1]   http://jackrabbit.apache.org/oak/docs/query/lucene.html#Suggestions

Thanks Rob. Please file an support ticket for further deep investigation & fix.