分析结构要从大处着眼，先找出基本框架，然后逐个细分。

What if.....? ——这是一个主句省略的说法。主句不完整。补充完整是：

What will happen if....?

What can they do if...?

What （主句：“他们该怎么办”）if later in life they come to a metaphorical door, like an opportunity of some sort, and instead of opening the door and taking the opportunity, they just stare at  it and wonder, "What if it isn’t a door?"（条件从句：“如果....”）

That is, "What if it isn’t a real opportunity?" （另外一句）

if 从句结构分析：

if later in life （时间状语）they come to a metaphorical door, like an opportunity of some sort, （第一个并列分句）and instead of opening the door and taking the opportunity, they just stare at  it and wonder, "What if it isn’t a door?"（第二个并列分句）

第一个并列分句：they come to a metaphorical door, like an opportunity of some sort, （door 的非限制性定语）

第二个并列分句：and instead of opening the door and taking the opportunity, （方式状语）they just stare at it and wonder, "What if it isn’t a door?" （wonder 的直接宾语）