Great news! I tried it again this morning - from scratch and with a new book - and it worked!
What was different this time around?
(a) I used a totally different book. I already had a copy of the first book I tried on my Kindle, though I changed the name when I put it through the Mobi2MobiGUI.
(b) I don't remember checking the boxes both times (before converting) when I changed the 'exth' type
(c) My wireless signal is a lot stronger now than last night
Whether it was one of the above factors or all, I don't know but it works! I'm really happy
And I didn't even bother to open the file with K4PC, I just sideloaded it onto the Kindle and the Blackberry.
AGAIN