First case:
內政部函【說明二】有關旅居國外4年以上且取得外國國籍之大陸地區人民,准予其身分變更為外國人,嗣後並得准以外國人身分申請歸化我國國籍。
“Explanation 2” in the MOI’s letter concerns a Mainland Area Person who sojourns abroad for 4 years or more and obtains a foreign country’s nationality, his/her identity changing to that of a foreigner, and afterwards may as a foreigner apply for naturalization to our country’s nationality.
It suggests – but doesn’t explicitly say – the person lost ROC nationality by naturalizing abroad. So I would say your way of putting it (“can be considered a foreigner”) is reasonable.
Second case:
依據「臺灣地區與大陸地區人民關係條例」暨其施行細則規定,大陸地區人民如果旅居國外4年以上,並且已經合法取得居住國的國籍,得持該外國護照申請來臺簽證。(僅限正式護照,持非護照之旅行文件如「旅行證」,則非我國簽證之核發對象)
I’ll just change one word in your summary.
A mainland citizen living abroad for more than 4 years and holding a foreign nationality should may use the foreign passport when applying for Taiwan visas.
And I’ll add the parenthetical sentence.
(This is limited to formal passports; the holder of a non-passport travel document such as a “travel pass”[?] is not the object of our country’s visa issuance.)