Q1: 少开车就得多走路,多走路就要少开车,虽然是两个短语,但是一个意思,是交通习惯的改变,所以就用does了.如果主语是more walking and less eating的话谓语就是do,因为是两件事.
Q2: 这是个倒装句,原来顺序是“More walking and less driving can not only benefit your health, but protect the environment.”这样看就好明白了,protect处于和benefit并列的地位,关联词是“not only...but...”.