How can I merge a content type's fields and settings from dev to staging?
To fix the issue include config in optional directory instead of install directory in your features module. It will then only install the config which are not present in the new site. Which will solve your issue.

When I try to enable to the module I get a screen full of &quot;unable to install&quot; because the configuration already exists errors.

Ok, I misread your question. To update existing config as well you need to do partial cim.
Copy the feature configs into a different directory named feature-config-sync (can be anything other than standard folder names) in default site config directory. And execute the below command. It will import new config and update existing configs if needed. Limitation is it wont delete any configs.

drush cim --partial --source=../path/to/config/feature-config-sync

IIRC you can even map the module install/optional config directory as source during partial import.
Once your import is done you can delete the feature-config-sync directory.