Sorry if this has been discussed--I tried searching forum numerous times and
could not find a definitive answer. I also Googled and searched the file,
but to no avail.

Can one select a layer and adjust the timing of the layer all at once? For
example, can the layer be made longer or shorter, proportionally changing the
keyframes? It appears that there was an old extension that did so, but I could
not find a modern one. I'm hoping it is a built-in feature that I'm just
overlooking.