Welcome.
1 - Are you talking about email addresses with which you've communicated that are now showing in your contacts? I don't see a setting in the GMail app but the option is there in the desktop interface, in the settings.
2 - I'm on a Nexus 6P but the apps should be the same. This is the banner at the top of my Contacts app (at the bottom of the post).
3 - I didn't use the Note 5. What I can say is that the Google and many other keyboards are are VERY customizable so you should be able to get a similar look to your keyboard. Was there something in particular that drew you to that keyboard?
4 - NP
5 - Not that I'm aware of. I suppose it might be possible with root but I don't play in that pool - I use my phone with corporate email and they don't take kindly to rooted devices.
6 - I have a 3ft Anker USB-C cable. There's a 6ft version (both are USB 2.0, though there is a USB 3.1 version of the 3ft cable).
https://www.amazon.com/Anker-PowerL...ag=hawk-future-20&ascsubtag=UUacUvbUpU5561471
I have a Vinsic car charger with a USB-C port, and for the times you need to plug in to USB-A, I have one of the iOrange-E cables.
When dealing with USB-C to USB-C it's not SUPER critical, but I'd stick to something from this list if possible (the sole USB-C with USB 3.0 cable in 6ft length seems to not be available anymore the Amazon link is dead).
https://docs.google.com/spreadsheets/d/1wJwqv3rTNmORXz-XJsQaXK1dl8I91V4-eP_sfNVNzbA/edit#gid=0