@fbartels i testet the ISO yesterday and it seem to be nearly full EWS. But there were some crashes and the support costs are nearly double of Kopanosupport.
the nice thing about a rest api is that since its webbased you are free to choose your local implementation language (you could even do it with curl from the command line). Either your ERP has a native functionality to call out external applications or you need some sort of middle ware (on the same server or a different vm/container/whatever) that connects to Kopano through the RestAPI and uses whatever api your erp system has to connect to it.
yes that sounds the standard mode of operation for fetchmail. Poll an external mailbox and deliver locally. If its really just a single mailbox for all your users then you likely need to deliver to a local mta who will then decide where to deliver which mail (the other mode for fetchmail would be to deliver directly to dagent, but then it would all land in a single inbox).
Maybe there is a magical way to migrate all users/db/attachments/settings over a weekend (this downtime would be acceptable)
Depending on the total amount of data a migration over a weekend should be no problem at all. You could already migrate the mysql database beforehand with a master/slave replication and copy the majority of attachments as well.
yes the ldap schema to manage Kopano through ldap is included in the source and therefore in all downloads.
Ldap Account Manager to my knowledge has a paid version that included templates to easier manage Kopano users, but as far as I know this does not depend on a specific Kopano functionality, but just interfaces with LDAP directly.