+1, see also Moving to a system font stack where the discussion was never really completed, but with 6 +1 for the proposal
+1, especially as it’s going to make the system font stack support more portable.
+1 for the same reasons
+1 as long as it does not hurt accessibility.
+1
+1 for the sticky footer options
+0 for the color change size, but I would only consider it if the sticky option leads to technical issues.
Also, does it have an impact on mobile where vertical space is scarce?
I trust you on that one 
+1 as long as it does not hurt accessibility
+1
I’m not sure to understand that one. If the unfold buttons are removed and clicking on the segment unfolds the tree instead. Are users still able to navigate to a given segment in a single click?
+0, I would need @surli opinion on that one.
+1
+1
+1
Is the text in black for links just for the mockup, or is it part of your proposal?