stylesheets-remove in .info.yml file doesn't work
I was also having trouble with this and I found a change record which states:


  "In cases where a Drupal core asset is being removed (for example, a
  CSS file in jQuery UI) the full file path is needed. In cases where
  the file is part of a library that belongs to a module or theme, a
  token can be used. Note that when using the token it needs to be
  quoted because @ is a reserved indicator in YAML."


In my case I was trying to remove the eu_cookie_compliance.css file from the eu_cookie_compliance module. To do this I used the following in mytheme.info.yml:

stylesheets-remove:
  - '@eu_cookie_compliance/css/eu_cookie_compliance.css'


To remove module (even core ones) provided css you could try something like:

'@system/css/system.module.css'


To answer your second question about the aggregation of css files, you can switch off aggregation by visiting /admin/config/development/performance and unchecking "Aggregate and compress CSS files"