I’d recommend the Stealth mode installation by dropping the User32.dll into the Steam directory
To answer your edit: It depends. Some DLC need to be downloaded seperately, you need to install those manually (most are downloaded with the base game in my experience). cs.rin.ru has many ressources on how to do that. You’d have to do that for CreamAPI aswell though. Adding non owned games is pretty much useless imo because you only get cloudsaves and achievement support iirc, you’d be better of playing a cracked version without Steams DRM instead.
The best thing about GreenLuma is it’s Family Sharing Restrictions bypass though. If you share libraries with some friends you can play almost everything together with only one license
The app is adfree and not spammy. Can’t talk about the Website though, I haven’t browsed the internet without good content blockers for the last decade