I\’m not quite sure jdavey quite understands the depth of the question.
SSL Certificates from Lets Encrypt, are issues on a 90 day cycle and require someone with SU access on the server to issue the installation command of sites that have been approved into its beta program (soon to be a public beta).
In this case, one of the Site5 admins would need to run a python script (there are several version available, they call them ACME Clients) to install and automate the installation of the SSL certificate.
The advantage to site 5 in this case is that it only needs to be set up once, and the script will handle the rest, automatically verifying and renewing the SSL installation on the server end.
Yes, these should be usable. As long as they contain all of the correct information, they should work fine on our servers.