"The quality of this shirt is good."
I think this is the most dangerous example of the ambiguity of 很 if it really only contextually means "very. " The quality of a shirt being good or very good will make quite a difference.
I would suggest, that in real life, if you want to say it's "very good," it'd be less ambiguous to say 非常好 or 真好 like in previous lessons.
If you want to say only "good," it might be safer to say 不错, which here would mean, "not bad."
Is the 的 here optional? I know one of the other problems had something similar with 质量 and didn't use 的 in the sentence. Is there a good general rule for when to use and not use 的?