I must admit, I was pleased to see that first question, about line height. I, too, learned to use unitless line height a long time ago, and I was surprised to discover that its behaviour is not widely known. A couple of years ago, I had somebody on these forums ranting at me for my utter stupidity in not redeclaring my line height after I changed font size, and my explanation was met with disbelief.
(It’s probably just as well he’s no longer around; I can have a quiet gloat without being tempted to remind him in person. )