Ah, I see the problem.  That second-to-last line should be "MNA 0,055".
 It has an extra "MNA" in there and the radio NAKs it with 'N'.  I
should be able to fix that here and shoot you an update to verify.
  

1) I cannot save the download memories.  The File --> Save and "Save As" 
are greyed out.  Maybe these options aren't used as there is Radio --> 
"Export to" ?
    

Correct.  "Save" means "Save an image".  There is no image for a live
radio, so you can't do that.

I've already added the aforementioned notice dialog, which explains
this.  You should see that in the next round.
  

2) By default, when downloading the memories on a THF6A, it downloads 
memories 0-399 but displays 0-25.  If I change the "Memories Memory 
range" to say
0-399 and click on go, it re-downloads  all 400 memories which takes a 
long time.  This re-download should be suppressed.
    

It's cached on some of the other live radios, and I can do the same for
the kenwood ones.  You are, I believe, the first person to attempt using
the kenwood driver(s) other than myself and a small circle of friends :)
  

3) If I try to edit an entry, say memory slot 55, and I type in a 
repeater name and then hit enter, I get a dialog box saying "Error 
setting memory: Frequency
0.000000 is out of range."  It then proceeds to re-read the entire 
radio's memory which take a while (this re-download should be 
suppressed.  The UI should either require the user to enter in the 
frequency FIRST or it should let the user enter in the memory's name but 
not upload the name it until a frequency is also entered in.
    

That has been fixed in the repository already, since the beta you are
using was posted.