Is there a way to test Engage without actually impacting the Sends?

Yes. If you want to enable Engage in your Marketing Cloud environment, and want to see what saturation control decisions it makes without actually having it impact your Sends, you can build a journey that follows a structure like this:

You can have the Oversaturated and Undersaturated branches join again so that the Send happens no matter what Engage decides.

This way you can see:

- how DESelect Engage builds customer profiles

- how it decides which contacts it included/excluded from which campaign Send (also referred to as just "Send/s")

- whether the rules that have been configured make sense or need tweaking

- get familiar with Engage and gain trust in the solution before having it impact your Sends

Once you're confident with your configuration, you can tweak the journey to follow a structure like in the image below: contacts on the oversaturated branch exit the journey, while contacts on the Undersaturated branch get the email sent.