Long-Term Resident Visa Clarification

The long-term resident visa states that people with Japanese ancestry are eligible for this visa. My Grandmother was a Japanese native before marrying my American grandfather and moving to the United States where she became a U.S citizen. My question is if I am eligible to apply for the long-term resident visa? Or since my grandfather was not a native Japanese citizen then am I not considered 3rd generation and therefor not eligible for the visa?

I plan on moving to Japan in a few years to attend a language school for two years. I know I can apply for the student visa rather easily for the two years but I'm planning on trying to become a citizen. I believe you need to have resided in Japan for a minimum of five years to even apply for naturalization so the long-term resident visa would greatly simplify that in the long run.

Also I do have all the paperwork tracing my lineage to my grandmother and her former Japanese citizenship. However I do not have any contact with any long lost relatives in Japan due to her marriage being less than accepted in her family at the time and her strong desire to be seen as an American. She avoided teaching us much of anything about our ancestry whenever my siblings and I inquired.

Grandmother was a Japanese native before ......became a U.S citizen...if I am eligible to apply for the long-term resident visa
Possible if you have documentation to proof it.Talk to Japanese embassy.

since my grandfather was not a native Japanese citizen
Got nothing to do with your grandfather.

I do have all the paperwork tracing my lineage to my grandmother and her former Japanese citizenship. However I do not have any contact with any long lost relatives in Japan

Again talk to Japanese Embassy.

Or read below
https://www.isa.go.jp/en/applications/procedures/index_zn3.html
https://www.juridique.jp/visa/nikkei.php