Overriding nested instances: what I would like to do is swap a nested instance for another. For example, I have a button instance and then an icon instance that is nested. I want to be able to swap the icon, which is nested. Sketch does this well, I've yet to see if this is possible in Figma.
Constraints that include padding: what I would like to do is to create elements closer to native web elements. For example, a button with 20px all around, I never want a button to break that padding. If I resize an instance using your directions above, I can make it really small/narrow and it breaks the padding.
Inserting instances: right now, from what I can see, the only way to add an instance is to make a copy of the original instance. In Sketch, you can use the menu and say Insert > Symbol > Symbol Name. Right now in Figma I have to search the whole page to find my instances, copy the instance, and then go back to where I was at and paste the instance, which is very inefficient.