If a Contact is deleted, will the Opportunities it is linked to be deleted as well?
No, the Contact will be removed from any records with which it was associated, i.e. the “Contact Role” related list entry will be removed from the Opportunity.
If an Account record is deleted, the Contact & the Opportunity will be deleted as well (although Accounts with a “closed won” Opportunity cannot be deleted).
When deleting Salesforce records with MassEffect, DemandTools utilizes the Salesforce delete code.
To test what will happen when deleting with DemandTools, manually delete a test record from the Salesforce user interface to see how related lists, etc. are affected.