I’ve done a reasonable amount of research on the issue of 1and1 + Let’s Encrypt. While there is conflicting information and I haven’t managed to reconcile the existence of all of it with each other, I feel safe saying that it is definitely possible. I was informed by my confused and self-contradictory customer service agent that my plan does not include Plesk access, but I’ve also seen it reported that manual installation is also possible on 1and1, which makes sense, as I, in theory, have shell access via SSH, as well as the ability to control and edit .htaccess.
But the customer service agent, who, again, seemed really confused, did try to tell me just now that externally supplied SSL certificates aren’t supported, which doesn’t make a lot of sense.
In any event, I’m looking for the straight-forward and up to date “how” to do the installation