Check your custom component for the selector which you are trying to use is not getting resolved. If u can add ur custom component code then we can help u out more 

Yes  you can.But how to access it depends on how the user details you are saving.

Whenever u create a keystore or truststore a node gets created in aeem .To change the password you nèed to delete previously created node in crxde and recreate through ui.

You need to write a custom workflow which will update the node where image is authored whenever there is update in dam.

Just tried on AEM 6.5 it works fine for me-  

Its same as we inherit any core component, so to reuse something u need to inherit the component and add the changes required for that component.

Hi, Could you please let us know any specific reason to place under apps?

Hi,yes it's recommended to have subdomain on same aem instance.Also u need to create separate farm files for each.

refer this one-https://stackoverflow.com/questions/31350548/resourceresolverfactory-getserviceresourceresolver-throws-exception-in-aem-6-1/31394583#31394583

Invalid Assertion: 1 2 com.adobe.granite.auth.saml.model.Assertion Invalid Assertion: Signature invalid. com.adobe.granite.auth.saml.SamlAuthenticationHandler Private key of SP not provided: Cannot sign Authn request Issue could be with certificate stored in the truststore. Solution here could be to...