We are going to start a closed BETA for the S/MIME plugin this week. Send us a DM when you want to join (not a lot effort required)!
You can get an Archiver license for as many users as you want, but minimum 20. The amount of Archiver users does not have to be equal to the amount of Zarafa users, but it must be less. So if you have a Zarafa license for 100 users, you can get an Archiver license for anywhere between 20 and 100 users (in multiples of 5).