You can't nest CSS rules as far as I am aware. So your double sets of curly brackets won't work.
Sometimes it is easier to just use basic coding, so I wouldn't be trying to combine rules
https://www.w3schools.com/css/css_combinators.asp
I would use two separate rules...
For all your standard paragraphs, use this for your 1em indent
p {
display: block;
margin-top: 0;
margin-bottom: 0;
text-indent: 1em;
}
Then for your first para after scene break, use the following. It will give you spacing above for the scene break. And yes, you will have to modify all the scene break p tags to include the class. <p class="scene">Blah blah.</p>
.scene {
padding-bottom: 0;
padding-top: 2em;
text-indent: 0;
}
If you want to use pseudo elements, they will need to be two separate rules anyway, and not all ereader software will understand it.
https://www.w3schools.com/css/css_pseudo_elements.asp