Child theme questions

Hallo, I’m new in this forum, just signed up two minutes ago.
I have some questions for the community, sure that s/one more expert of WP has the replies that help me (thanks in advance).
This is my first site built in WP system: till now I built-up sites just writing all codes by hand.
I sound good, but a couple of points are still unclear for me:

1) I started from a child-theme of Twenty Seventeen, but step by step I totally re-wrote the theme: not in the main theme sheets (not touched) but applying external sheets (mainly css, htmland js, but also pho and jquery) …
till today the updates of TwentySeventeen were automatic, but this time it asks me if I want the update: probably this new update would not touch my re-coded site, but I don’t want to take the minimum risk, so did not accpet yet, and the reminder is always there.
Of course I backup my site, but the point is:
if I install a new update, and then want to revert back to the previous version:
IS IT POSSIBLE? and if yes, is it complicated or simply roll back by some WP option?

2) even if I try to keep the site cleaned, I see (looking inside domain with a client like FileZilla) that “all remains stored in the site folders” …
I have an unbeliavable number of ttemporary autosave’s, all images which I uploaded since last October are still there, EVEN IF I COUNTINUOSLY APPLIED “delete definitively”, I uploaded dozens of test files, changed time by time, and all my drafts are all there.
The site is “blown-up” to a huge heavy even if my effort was to create very light solutions: practically the site is 600% of its dimensions cause of all these waste files.
I know that I could delete all with some plugin, or going inside, but this is INNATURAL and some accident could happen (*read below*):
“why my trash is not working?” “why I cancel images on December 2017 and they are still inside folders today?” (of course not visible, but permanent in folders).

(*note*: f.e. I use the media library as a storage folder for many items which are not directly linked in pages: so if with a plugin I just say “delete all what is not aligned with WP site”, the plugin would cancel many useful files which I need for external pages).

These were just the first two questions, but since the story is becoming long I stop here for the moment:
hope to read s/one’s help.
THANK YOU!
Mauro (Vicenza- Italy)