Child Themes / Theme updates / Custom CSS

Hi, I’ve just lately got to grips with child themes (after realising i should have used them before with quite a few sites unfortunately), I understand moreover why i should use (and do now) but have a couple of unanswered Q’s..

1. Is it possible after customising the actual theme (using the themes custom css or a custom css plugin) to create a child theme from it so that i can update the theme without loosing the customisation?

2. If i have used the themes own css customiser do i loose this when updateing themes version?

3. I understand about placing the same css file in the child theme that you are changing, but i usually use the themes custom css box if there is one. Recently i tried to change some css on a site regarding responsive code, namely the widths where it changes to scale. I find this very confusing regarding child themes css / a themes custom css, (as you cannot change the media queries in the themes custom css?) if there is any info anywhere regarding, a link would be greatly appreciated as would any help with the above.

Thanks.