Tool for merging records

You will probably want these set in your production application.  However if this is a Sales app, your sales team might have a strong opinion on requiring the data.

To your second point, new records after you go live will probably need a different process.   Cleaning full data sets for initial load is not the same as adding a record that will take on life cycle from scratch.  You need a data validation strategy for new records so that when they get entered you have new, non duplicative info.    Assuming that everything starts with a Contact and there is a business process where the Case and Item develop over time, requiring unique email addresses might solve it for you.

Here is another strategy.  I have got a client with a large sales staff.  People constantly come and go.  A couple years ago it became clear that starting their data in the main system was a disaster.   We created a dedicated Onboard Offboard application for managing people.  Once the work flows were complete there validating a bunch , then the people are entered into the main system by the Ops team.