@necrevistonnezr Great questions! I'm still pondering this myself, but got delayed with some other projects.
I don't have an HA setup yet, but I'd assume the easiest way to achieve it may be to duplicate the server (i.e. restore Cloudron backup on a new server), then use rsync to keep them in sync on the boxdata directory for example, then use DNS for failover where the MX server address on the other server can be used with a higher weight to be last in the selection and thus only used if the first MX server isn't responding.
Just my initial two cents anyways, but haven't thought it out thoroughly yet. Will probably give it more thought once the Cloudron multi-server management feature is in-place as it may make things a little bit easier.