第一句是倒装.
但第二句有一点：当only强调人,物,代词,名词时不倒装．而且不能用在否定句子里.第二句强调 a doctor,不倒装.这一句实际上是after they came in,there is only a doctor (that/whom) they saw.
这是个人理解.供参考.
关于补充问题：
what do u like?
---Pingpong.What fun it is to play in the open air.
【我觉得这里已经错了.what + 名词,正确的应该是What a fun it is to play in the open air.】
然后可以换成：how funny it is...【how + 形容词】
fun是名词,funny是形容词.