"it appears that every time a new LR version (not minor update) is installed, a new catalog and preview folder are created."
Not quite. When you install a new version (not dot release), it detects the catalogue which the previous version would have opened. It then copies the existing catalogue and adds -2 to its name, leaving the previous version's catalogue untouched. So from now on you'd be working with Alan-2.lrcat, for example. With the previews folder(s), it simply takes over the existing ones, adding -2 to the name - eg Alan-2 Previews.lrdata. This name change ensures it remains associated with the new version catalogue.
You can get rid of the old versions. Back them up first, of course.