And whereas his was once a lonely if plummy voice crying in the wilderness, the Prince has seen many of his once maverick opinions become mainstream. 

你对句子结构理解错误。if plummy是补充语（你可以理解为插入语），是形容词lonely作补充说明。His was a lonely voice. 他的声音是孤独的声音。加上补充语if plummy, 则为：His was a lonely, if plummy, voice. 他的声音是孤独（尽管有些拿腔拿调）的声音。crying...现在分词短语为voice的定语，与if从句没有任何关系。