Most VRAM is fine, even reballed ones are usually ok if it is not Samsung from 2006-2008. SEE is usually ok with their chips.
Stop messing around with not native chips. I bet if you reball the old GPU it will work just fine. Compare strapping resistors like R8611/12/13 between 0084 and 0005 boards. You cannot just slap different chips on there and expect it to always work. It should however at least produce some sort of image......
I honestly don't exactly know. I never took the time to figure it out nor will I ever. I suspect a BIOS issue. 6770 was only used on late 2011 models so it will probably never work on early 2011.