首先你们老师讲的‘that的定语在后面,one的定语在前面’前提是用一个单词做定语的,且that 表特指,one表泛指.
在你所给的第一个句子中（正确句子应该是：One of the most important questions they had to consider was___of public health.）,that 指代前面的question. that of public health =question of public health.
第二个句子,one 指代一件礼物a present ,为避免重复.因其后面是介词短语做定语,只能放在one后面.