I also find them a little strange. Grammatically speaking, I think they are incorrect. I would probably write them as something like...
I would describe 「V した＋ことがある」as to convey having a certain experience determined by the verb. In English it is usually rendered as "have done something". In much the same vein, 「V した＋ことがあった」also talks about having that experience, but the having itself is talked about as in the past. With あった there is a distance between that experience and the ...
態度 / と / いったら / なかった/。
態度 == attitude
と == (i don't know how to explain)
いたっら == is
なかった == bad (in only this sentence)
normally ”なかった” meaning is "nothing" and this is past participle.
"ない" is now participle.
Saying 毎日学校に行ってくる implies the "mental point of view" of this sentence is fixated to the speaker's home, but I think that's weird. A sentence like "I go to school every day" is usually used outside one's home, and returning home is not really part of the purpose of this habitual action. Compare this with ママは毎日スーパーでリンゴを買ってくる, which is ...
美味しいでしょう is a natural and standard Japanese to say "it’s delicious, right?". (I'm not sure what you mean でしょう as probably so far.)
美味しいじゃん? is basically the same but it's a colloquial form. But I can imagine "it’s delicious, right?" in the meaning of "Please agree with me it's delicious?" or "I'm very surprised it's ...
I asked my wife, a native speaker about your question, and she agreed with you. She said 誰でも is actually correct in your original sentence because it is affirming (肯定文) in your statement. 誰にも, on the other hand, would imply the opposite, a negative statement (否定文), and hence sounds strange in that context. 自明の理 requires agreement, something that is known by ...