You should be able to achieve this  with mix of etc mappings to map/rewrite content path and rewrite rules to handle the extensionless urls.  Have a look at (1) to get an idea on this.(1) : Extensionless URLs with Adobe Experience Manager | Olson Digital

You'll have to adjust the xss config to avoid the antisamy configuration to strip off these attributes.Once you click save, Check the error.log that would provide the message that would give you an idea that antisamy is at play.To fix this: You can follow (1) on similar lines.(1) : XSS Filter issue ...

Can you enable debug logs for the search and help me with the query that is generated at the end? Maybe we can somehow tune the index definitions and make it better performing ?

This configuration of max 5 version and max age 30 related to to version purge i.e deleting versions which are not in use. Only if you run or  schedule a maintenance task to purge versions this would come into play.There is no finite limit on how many versions you can create in the system.

Amazon S3 Data Store and Azure Data Store are the only supported implementations supported for external datastore for AEM for now. See (1) for more information.(1) : Configuring node stores and data stores in AEM 6

This is a case of 2 concurrent threads writing to the same node. Setup a debug logger for 'org.apache.jackrabbit.oak.plugins.commit.ConflictValidator' and identify these 2 operations. It could be a launcher, observationManager etc. and there would be a race condition we need to fix.

Yes This is visible from the url (1) which we missed. Glad it worked out in the end.(1) : http://dev-author.westeurope.cloudapp.azure.com:4502/mnt/overlay/myapp/components/widget/c%20ontent/pathfield/picker.html%7Bvalue%7D​

By default, if you don't run the version purge there isn't a  finite limit to the no. of versions of pages/assets  to be retained but that might affect  overall system performance Once you run the version purge, it then picks up from the config and by default it would 5 versions that are 30 days old...

15.05.2018 14:31:39.784 WARN [81.128.191.42 GET /editor.html/content/myapp/undefined.2.json HTTP/1.1] com.adobe.cq.xf.impl.servlet.rendercondition.EditorRenderCondition Page at path /content/myapp/undefined.2.json doesn't have a jcr:content nodeWhy is it triggering an undefined for your myapp and p...

There should still be a config 'Apache Sling Java Script Handler' for AEM 6.3 as well. Can you check what are the value for Source and Target VM ? Change it to Auto if it is something else.Also, Can you post the complete error.log and I can review it for you.