How about a check box to confirm that you read the message... then an 'OK' click is thwarted, and people are forced to read.
Usually I'm opposed to this sort of GUI design.
However, I did briefly consider a checkbox for "yes, I posted complete source to reproduce my problem" on this forum, which would enable the button to post a new thread. But simple adding guidelines and the "forum rule" with red text seems to be working pretty well. (I recently spent a little time over on Arduino's official forum.... we have much better quality questions and most thread arrive at good answers much sooner).
But, back to the installer GUI.
I do like pawelsky's list of words.
1. SKIP
2. ADD
3. REMOVE
4. KEEP
5. UPDATE
I think these might work well, perhaps also with use of color, probably gray for SKIP and KEEP, red for REMOVE, maybe green for ADD and UPDATE. I'm imagining the "all" button will turn them all green (ADD the ones currently missing, UPDATE the ones already installed), and "none" would turn them all grey. To delete already-installed libs, I believe clicking on each individual one is probably reasonable.
Just to be absolutely clear, this conversation is about possibilities for a future installer. Maybe 1.21, but probably farther out. A big last-minute change is definitely not going to happen in 1.20, which I had intended to release today... but I'm investigating a mysterious issue when OctoWS2811 and Audio are used together in a specific way. I want to fully understand this and make sure it's not an obscure bug before a final release.