Email Issue with Squarespace Domain

I have a Squarespace domain and email hosted by a 3rd party (inmotion hosting). My email host told me there is an issue with Squarespace Nameservers, which have two different sets of NS records. One of those sets needs to be removed. If all the DNS records are with nsone.net, then I need to remove the Squarespace NS. Also, adding Squarespace NS and not removing the other is causing conflicts. "You have two different sets of nameservers, so your domain can query either NS for its records. If you are using Squarespace NS for DNS management, you must remove the other ones". 

Squarespace confirms "those nameservers that you were advised to remove are Squarespace nameservers and they cannot be removed unless you chose to use custom records".

I am not sure what to do to get my email running correctly. Can anyone advise on what I need to do?

Certainly not an expert here but have you tried using the custom option and simply entering one of the "default" Squarespace NS groups (i.e. the 4 SS ones OR the 4 "nsone" options)? I don't know if it will allow you to set technically "default" nameservers as "custom" nameservers, but you could essentially skirt the problem if it does.