Is it necessary to use your modules to implement Doofinder?
No, it is not necessary. If you have a product data feed, e.g. Google Shopping; that's enough to get you started.
We couldn't check the plugin due to a connection error
Doofinder tries to check the plugin installation by a URL. If Doofinder cannot access the plugin, the guided configuration can continue. Just select the "other eCommerce" option as a platform and proceed with the configuration.
How to test Doofinder on a Staging Website?
Yes. Create a separate store in the Doofinder Admin Panel, copy its unique installation script from Configuration > General Settings > Installation Script, and and paste it into your staging site's HTML. This keeps your production setup untouched.
See the step-by-step here!