1) a)
~するだけは~したほうがいい is a set phrase, meaning "it's better at least to do/you should at least do". A similar expression 話だけはしたほうがいい makes sense, maybe?
だけ modifies "to talk", not teacher, so it doesn't connote "even if you don't talk to anyone else". See the difference between 先生にだけは話したほうがいい and 先生にも話だけはしたほうがいい.
b)
Yes.
2)
どの程度であっても?
a)
by/within
b)
It's more likely "rephrasing"; その人(物)ができること, i.e. 能力.
c)
What the writer wanted to emphasize would be プラスに評価して述べる. For instance, 彼なりにやり遂げた does not just mean "He completed it in his own way/style", but also connotes "even if his way is not like what expected, further, even if his way is worse than the way others usually do, the speaker values it positively". In fact, なりに sometimes has this "worse-than-the-way-others-usually-do" nuance.
3) a)
Your translation "How many should I get (for you/us)?" is correct, but 買ってくれば is from 買って来る.
-ba conditional
くる --> くれば
くれる --> くれれば
b)
できるだけ is not incorrect, but in the form 1万円で___だけ____, it's common to interpret で is used as a particle here. できる is a single word, not "で + きる". What the questioner expected is その1万円で買えるだけ買ってきて, for instance.
4) a)
Yes, the parents went out having fun.
b)
leave their child home alone (without taking care of him/her)