I'm sorry to hear that.
I do have one last suggestion. There is something I want you to try. Use Notepad or Wordpad to open one of the nonfunctional ebooks. Search for BOOKMOBI , and replace it with TEXtREAd . Then save the ebook. Tansfer it to your ebookman and see if it works.
This may or may not work. If it does, then you have a simple way to hack the ebooks so they work. If not, then you will need to look into the Mobipocket tools around here. You'll need to disassemble the file to it's HTML, then put it back together.