Does my bootloader uploader upload a bootloader OK? If so, then after that are you saying the (new) bootloader fails to upload a large sketch? That sounds then like the hex codes for the bootloader might be wrong. If we replace them with the bootloader that works (like WestFW's one) then things should be OK.
Correct. I have an incredibly stupid question. With the Atmega Board Programmer, the 1284p h file (bootloader is 1024 in size) is ~4kB, while the 1280 h file is ~24kB (4096 in size), is there any reason for this? I see the 2560 h file doubles the 1280 h file size.
It really would be nice if someone could build out a 1284p on a breadboard and burn a standard optiboot or optiboot mighty 1284p with the current ABP software and with an arduino. Then upload through FTDI (or even a aduino serial) the ABP program and report the results. And, if that works, pull the hex out of that chip and post here or compare to the current ABP?
The optiboot atmega1284 UART1 is good to have as a stopgap, but it really would be nice to have things a little more standardized when working from anything Arduino.