英语长难句分析 If its message were confined merely to information—结构分析

If its message were confined merely to information—and that in itself would be
difficult
if not impossible to achieve，for even a detail such as the choice of
the colour of a shirt is subtly persuasive—advertising would be so boring that
no one would pay any attention.
这句中and that in itself...这句怎么理解? that指代什么，是非限制性定语从句吗？请也把全句结构帮忙分析下，谢谢！

英语长难句分析 If its message were confined merely to information—结构分析
回答：
你所提供的是一个较长的主从复合句，其中有2个if 引导的条件状语从句，其中还有一个 for 引导的原因状语从句，一个so...that...引导的结果状语从句；
1）If its message were confined merely to information：条件状语从句；
2）that 是指其前的主语message；
3）and that in itself would be difficult：这是第一个主句，其语法分析如下：
that---主句中的主语，并非指代非限制性定语从句的；
would be---系动词；
difficult--表语
in itself---adv. 在本质上；完全地；本身；
4）if not impossible to achieve：第二个条件状语从句；
5）for even a detail such as the choice of the colour of a shirt is subtly persuasive：
原因状语从句；
6）advertising would be so boring that no one would pay any attention：第二个主句；
7）so...that 在此句中引导结果状语从句：如此...以致...；
8）全句的英译汉如下：
如果其信息只被局限于信息和其本身的话，那是完全难以做到的，如果能够如愿以偿的话，因为即便选择衬衫颜色这样的细节也具有细微有说服力---那么做广告将会是如此无聊，以致没有人会搭理它了。

说明：若要更正确地理解全句，最好能把该句的前后句子（即上下文）都写出来，这样对理解本句会大有帮助的。欢迎追问！

这里的that不是定语从句，that是指示代词，代指前面的句子，也就是its message were confined merely to information这句话。 and that in itsef是指”这件事情本身“。

If its message were confined merely to information—and that in itself would be difficult if not impossible to achieve，for even a detail such as the choice of the colour of a shirt is subtly persuasive—advertising would be so boring that no one would pay any attention. 如果广告仅仅限于提供信息，那么就太枯燥乏味了，没有人会留意它。这样做即使并非不可能但就广告而言很难，因为甚至像选择衬衣颜色那样的细节也是具有微妙劝诱性的。 advertising would be so boring that no one would pay any attention If its message were confined merely to information 主干。 —and that in itself would be difficult if not impossible to achieve，for even a detail such as the choice of the colour of a shirt is subtly persuasive— 同位语