I needed to soft reset my Kin recently due to some random crashing (which I believe was caused by a long text message I was unable to delete). I wanted to backup my contacts and import them after I cleared the phone. I used the same procedure you mention here expect that I downloaded the contacts_export file created by using the "Export Contacts" button on the developer menu.
I renamed the file from "contacts_export" to "cellebrite.contacts.xml" and uploaded it to the phone. After a restart I tried using "Import Contacts" but continually got error messages about Invalid XML. I have spent a decent amount of time trying different things, but I think I have narrowed down the source of those errors (at least in my case).
Each entry must have a first and last name. If you had any contacts in the KIN without that they are exported without a <LastName> tag, and this causes errors during import.
A simple solution for this is to open the exported file and add in empty tags <LastName></LastName> after </FirstName> for any entries without a last name. The same could be done if any of your contacts didn't have a first name.
Once I did that, I was able to use JohnKussack's tool to upload "cellebrite.contacts.xml", restart, and "Import Contacts".
Just another thing to mention, all of my entries only have one number (mobile), a First/Last name (after I fixed them), and nothing else.