And there are examples:(新完全マスター文法N3より)
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ほど as much/to the extent of... アメリカではフットボールほど人気のあるスポーツはない (no sport is as popular as football in America)
ぐらい about/around 午前７時ぐらい起きます (I get up around 7am)
だけ only それだけ大丈夫かなぁ？ (is it ok with only that?)
１。２ 医者にお酒を止められているが、ちょっと(。。。）いいだろう。 Even though (が）the doctor made me stop drinking alcohol (医者にお酒を止められている), .... a bit is ok, right (ちょっと。。。いいだろう)?
The logical answer would be "ONLY a bit is ok, right?"
So だけ is used, and since it is contradicting the last clause of the sentence (she/he is going against the doctor's wishes), you use でも (but) instead of では.
The doctor made me stop drinking alcohol, but only a little is fine, right?
(I) met (her/him) only once... What (どんな） sort of person （人物）he/she is, I don't know.
In this case, it is because the subject ONLY met him/her once that she doesn't know what type of person that is.
Since (unlike the previous example) the first clause supports the following one instead of going against it, I would use だけでは instead of だけでも。
I only met him/her once, so what kind of person he/she is, I don't know.
I'll try my best to answer.
だけでは is just begging to be completed with ...ない. So it doesn't quite fit there.
The も in だけでも is acting similarly to how さえ in the grammar pattern Xさえ...eば functions. So the も behaves somewhat like "is enough for." For instance "Just thinking about that is enough to make me feel embarrassed" それを思い出すだけでも恥ずかしい or even better, "just doing my normal school commute is enough to make me tired" 通学するだけでも疲れる。 I'd say it doesn't fit here because it's a bit strange in the same way that it's strange to say "a little is enough for being good."
I don't even know what would have to possess someone to choose などなら.
Remember how I said that だけでは screams for ない? I would choose it based on that. Not sure how to disprove the other options (other than だけでも, where the same reasoning from the first question applies).