1°) This is just the Android way of doing things.
You’ll have to live with that, unless somebody here could name an additional App which perhaps can add what you want as an overlay which can close the currently used App with a tap. I don’t know something like that, and Android Apps usually use the complete screen real estate including the corners, so how somebody imagining such a feature would go about realising it I can’t really see. Perhaps with a button combo.
2°) I’ve only seen such weird phone number formatting in contacts added in Signal by Signal itself so far, not in Contacts and the dialer, but I sync all my contact data to my phone from Thunderbird on my PC, perhaps the reasonable number format I see in Contacts and the dialer just gets imported this way.
I see no setting for the phone number format used in Contacts. If you edit the numbers to look reasonably formatted to you, do they stay that way afterwards?
3°) No idea.
4°) Not that I know. It looks to me like battery saver capabilities are designed to help you to keep the phone alive until you can charge it again, they are not meant as a constant setting because they keep the phone from functioning fully in its intended way.
To get near to what you want without any tinkering the system doesn’t expect and could react badly to, you can set the Performance profile to “Efficiency”, which among other optimisations will already reduce CPU power (as per the description displayed for it), or to “Power save” for maximum power savings.
5°) As you already know you need root for a chance to control this directly, without root you can have a look at the Performance profile instead (see above).
And comparisons to completely different devices from the distant past won’t get you anywhere, don’t expect this battery life from current smartphones if you really use them. Just having them idling around in standby for days is possible, though.
6°) No idea.