@dezu As a general rule you want preferably to store your backup somewhere else. If you keep it on the same server, it really only protects you from everything software related directly linked to the programs, or a mistake like a deletion. You have no protection against the hard drive of the server breaking down, a fire, flood, a virus affecting all files, a mistake on the server level (deletion of everything or reformatting for example), etc. As we usually use VPS providers, you don’t know what could happen. They can go bankrupt one day and you may not have access to your VPS anymore, they can get angry at you for whatever reason and kick you out (that shouldn’t happen without at least a notice, but hey…), or they may get robbed and some thief stole the server, etc.
Ideally, you want to make regular copies where it is unlikely an incident would affect the two locations at the same time. It’s pretty simple these days, just use for example a cloud provider like Dropbox, Mega, Google Drive, etc. Encrypt your files before putting them there if you want.
About 2., yes definitely. You can do all that. There is usually no reverse proxy involved in any of this, I guess you meant reverse DNS (a reverse proxy is the opposite of a proxy, it’s a bit like a proxy for a server instead of for a user. It would for example allow to mask the server IP by giving the IP of the reverse proxy. No real use for any of this. A reverse DNS is what Daveteu described).
There are 2 things: Sending mail and receiving mail. The receiving part is pretty simple. All you really need is MX records in the DNS of your domain: That will tell where to route emails sent to email addresses@your domain. You can either configure this manually in the nameservers (NS) of your registrar, or use the BOX as NS. It is recommended to use the box as NS for the box itself (primary domain), but for additional domains, it isn’t really necessary, at least for the receiving part. If you do it manually with external NS, you just have to enter MX record pointing to your BOX on the primary domain. There is no need for any reverse DNS to receive mail.
The sending is a bit more tricky as you don’t want your emails to be recognized as spam, or worse, totally rejected. Here, it needs a bit more configuration, and is easier done when you use the BOX’s internal NS which will configure everything automatically. You can also do it by configuring the NS of your registrar, it’s really some copy/pasting, the box giving you all the information to enter. But what is recommended, is to not send directly the mail from the the secondary domains, but to route it through the primary domain (the box itself), which will do the sending. If you use the webmail or the SMTP server, or if the same machine handles the other domains, this will be done automatically. If you have some email sending from another machine/VPS/IP, you’d better configure it to not send directly but route through the primary domain. This is described in the “advanced tips” section of the mailinabox.email website: https://mailinabox.email/advanced-configuration.html . You don’t need to configure any reverse DNS on additional machines/VPS/IP if you route through the primary domain (so you only need one reverse DNS for your primary domain, that’s all).
It isn’t as complicated as it may look at first. Anyway, you can probably achieve everything you wish to do for multiple domains and multiple addresses.