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It's from 良し, which is the dictionary form of 良い ("good") in classical Japanese. It's just like saying "Good" or "Okay" before making a decision in English. よし is now considered a lexicalized interjection, but 良し is still used in modern Japanese in the sense of "excellent".
5
They are not interchangeable.
耳が痛い is used after hearing a righteous criticism about you. It's an idiom that means "It's a tough remark but I must admit it's true". You don't have to hear it more than once. The first three authoritative dictionaries say nothing about repetition, do they? I don't know why some sources say 何度も, which I believe is not ...
1
I have it in my dictionary, so I care share this interesting idiom:
みみがいたい 【耳が痛い, 耳がいたい】
表現 (句, 節, など.), 形容詞, 慣用表現
ⓐ being painfully-true (e.g. reprimand), making one's ears burn, striking home (e.g. remark)
ⓑ having an earache, feeling pain in one's ear
Source: 白檜辞書
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