I am unfortunately not able to contribute to the flashing problem.
But regarding the broken USB-Port:

In case you do not get it fixed at Nokia NZ, it COULD be possible to fix it for somebody who is relatively skilled, IF the PCB is not broken and the soldering pads are still there. As there is no warranty anyway...

Search for somebody skilled with a soldering iron plus the N900 Level 1+2 or Level 3+4 Service manuals in the internet. There are instructions on how to disassemble the N900. If you take good care, you should be able to get it fixed by a skilled person.

I am expierenced in soldering and would try to do it for you, but I think its a loooong way, unfortunately Again, I do not want to encourage anybody to break his device. My first answer would be 'send it back for warranty repair'. But in this case...

Best regards,
Corwin