I assume you mean backups of your configuration. The best way to do this is to copy the config.json and keymap.json files, as well as the templates folder if desired, from the following directory:
- On Linux:
~/.config/Mailspring(or$XDG_CONFIG_HOME/Mailspring) - On macOS:
~/Library/Application Support/Mailspring - On Windows:
C:\Users\<You>\AppData\Roaming\Mailspring
Those files can be moved into that same directory on another system to restore your settings, accounts, and templates. (You’ll still need to reenter your passwords on them, of course.)
We have an open feature request for cloud backups of Mailspring’s config file. If you want to see that feature implemented, I recommend you vote for that issue: Sync templates, signatures, accounts etc accross multiple devices
As to your other question, to my knowledge (I’m just the community manager, not the primary maintainer), there is not presently a feature to view all emails in plain text only, although you could request such a feature.