Hi Alex,
When you mention a "Salesforce Org", are you referring to a different Salesforce account? If yes, then Cvent does allow you to change which Salesforce account you are connected to.
With the out of the box integration, you can do this by entering the new Salesforce credentials in Admin > Integrations > Push API Integrations > Salesforce. Alternatively, you can choose to purchase a new integration. If you'd like to start a new integration, I'd reccomend reaching out to your Account Manager for pricing.
In regards to matching by Contact Stubs, we typically don't see those being used unless you have an API. With the out of the box integration, the way Cvent identifies Salesforce Contacts is based on standard and custom fields. If this is the out of the box integration and these fields are being migrated, Cvent should be able to recognize them.
Lastly, the integration should work with the NPSP instance so long as you have API access in Salesforce. However, I'd double check with Salesforce to ensure the package isn't missing any key elements.
Best,
Katie d'Esterhazy
Cvent Client Services