Re: Nested Comments are not indenting
it’s not a CSS issue the markup is malformed, trying to correct malformed markup with a presentations language is not a good idea CSS is predicated on well formed DOM structure it’s a symbiotic relationship if one is broken so id the other.
If you really need to fix quickly and are comfortable then access the comment function in the bp-default themes function.php file remove it to a functions file
in a child theme and look for the last closing tag and delete it, then comments will correctly nest within parent li elements. naturally then you’ll need to add any specific styling you want via CSS.
As the fix for this is in place and will filter through when versions are upgraded it would be pretty safe to edit the bp-default themes functions file to remove that one tag, but only this one time mind; as always though on ones own head be it! 
