A former PRC national has ROC nationality?

[tando]

You may already know, but for people who would visit here to look for information.

Pros and Cons of visa-free long-term stay in Taiwan (aka what are my options?)

Notices for the submission of application of Hong Kong and Macau citizens for residency in Taiwan area (TARC) in Chinese 香港澳門居民申請在臺灣地區居留送件須知 https://www.immigration.gov.tw/5385/7244/7250/7296/居留/148063/

And

ABT (36+ years old) w/Dual Citizenship Parents (Taiwan/US) wanting to become Dual?

Actually in this case, Taiwan immigration would not view her as a HKer, because she also has a foreign passport. Check out this immigration page that talks about the process for applying for a TARC as a Hong Konger - they ask you to sign a document saying you don’t have any foreign nationalities or passports.