Well, your NAND's block size limits our options, but with the new bootloader I'm not sure if it really matters tbh. For now your best bet would be to flash the stock ROM and wait for someone to crack the hboot you have in place. HTC has gone through great pains to keep the newer ones from being overwritten with S-ON and any more tinkering with it will most likely leave you with less options than you currently have.
I put the KDDI stock ROM in a nandroid folder so you should be able to use this and restore the phone. All you have to do is unzip it and then move the entire folder to your sdcard under .\nandroid\HT?????????\ . Once it's there you can restore it like any other nandroid.
Keep in mind that with your phone in it's current state you will not be able to accept any OTAs; in order to upgrade the ROM you will need to flash it manually through the recovery. Although, if a newer KDDI RUU comes out, and your CID was not changed to Sprint's, you should be able to flash that one and have it completely restore your phone.
http://dl.dropbox.com/u/18734200/BS-KDDI_1.05.970.1_stock_unrooted.zip