Use preinstalled service feature or not?

This is really too vague. There is no best practices, there is business requirements and how to satisfy them. 

Sorry for that, I read that using preinstalled service required modifications on the DC after which admin perms are not required for the QMM account. Is there any other benefit to use this service?

The benefits are you can migrate passwords and sidhistory with the need for an account with membership to the builtin\administators group. 

I think most of your questions can be better addressed by our Professional Services team. From a previous message I understand you were already contacted - still, if you need further assistance, email me at my name dot my lastname at quest dot com and I'll put you in touch with someone who can help.

I think most of your questions can be better addressed by our Professional Services team. From a previous message I understand you were already contacted - still, if you need further assistance, email me at my name dot my lastname at quest dot com and I'll put you in touch with someone who can help.

Hi Alex, I've send you an email. Thanks for your support!