1.simple as the tales were是独立主格结构,作条件状语从句.相当于as the tales were simple.尽管故事很平淡
2.逗号是可以连接句子的啊,你说的是单句的情况吧.在并列简单句,复合从句中,都是用逗号隔开的,这个句子整个是一个复合句.People liked his stories是主句,有主谓宾结构,because 后面的全部内容做内原因状语从句.在这个原因状语从句中,又有simple as the tales were作的条件状语从句,和to the reader’s surprise作的结果状语从句.they would finish with a sudden change at the end则是原因状语从句的主谓宾结构.