Photos app on iOS question

A few quick(ish) queries I hope someone can clear up.

1. There doesn’t appear to be a way to view titles and descriptions for photos synced over from Photos on Mac, is that correct?
2. If I turn on Photo Stream and/or iCloud Photos, there doesn’t seem to be a way to stop screenshots and images saved from the web from being included in the Photo Stream or iCloud Photos - is there a way to prevent this, or to filter them out afterwards?
3. If I turn on iCloud Photos on my iPhone, what happens to photos I take on the iPhone. I understand if I delete them on the iPhone they also get deleted from the Cloud, as it syncs rather than back-ups. So, presumably my phone’s storage will get increasingly filled with photos/videos taken on the iPhone, and I won’t be able to delete any if I want them to remain in iCloud Photos? Or if I chose the optimise storage option, do the full size photos get replaced by optimised versions at some point?
4. I noticed there was a setting to optimise storage on the iPhone - does anyone have any idea how much space the optimised photos take up on an iPhone. As a rough example my Photos library on my Mac is around 20GB in size, roughly how much space would this take up on the iPhone.

Thanks in advance for anyone’s help.