otherwise在从句中与虚拟语气搭配使用的问题
一个本来很简单的问题被你搞的这么复杂。如果你查一下词典关于副词otherwise的释义，你就会知道otherwise有三种用法，其中之一就是用来表示如果前面所说的事情没发生，或者情况不是前面所说的那样，结果会如何。常可将otherwise理解为一个条件状语从句，if not. 例如：
My parents lent me the money. Otherwise, I couldn't have afforded the trip.
= If my parents hadn't lent me the money, I coouldn't have afforded the trip.
Shut the window, otherwise it'll get too cold in here.
= If you don't shut the window, it'll get too cold in here.
从以上例句可知，otherwise隐含否定意义，相当于一个条件状语从句，但主句并没有否定意义，主句是根据这个否定条件句推理出的结果。
网友的句子均属于otherwise的这种用法。分析如下：
This might sound small, but to undo the effects of such a decrease a candidate would need 30 more GMAT points than would otherwise have been necessary.
这个句子中的than比较分句为than后接谓语动词。网友对这个than比较分句理解有误，才产生了疑问。
首先我解释一下than分句。我换一个结构相同但更为简单的句子说明问题。
几乎所有语法书都认为than引导比较分句时是连词，不作成分。比较分句除了经常采用缩减形式外，主句比较项的对应项在than从句必须省略。例如：
To travel first class, you will have to pay more money than is otherwise necessary.
than从句缺失了主语，补全后的比较从句为：
To travel first class, you will have to pay more money than it is otherwise necessary that you should pay.
在这个补全的than从句中，it为形式主语，that从句为真正主语从句，从句谓语should pay后的宾语为主句比较项more money的对应项，必须在than从句中省略，即than从句中的谓语动词pay缺少宾语。（比较分句在结构上是不完整的分句。）
由于比较分句中的that从句在结构上与主句相同，则通常省略that从句和形式主语it，得到than is necessary. 表面上看，than是从句的主语。（有人认为than在从句中作了主语。其实如果认为than是比较代词，在从句中作成分，than为省略了的that从句中的pay的宾语。）
这句话的意思是，你乘坐飞机旅行，必须付一定数额的机票钱。如果你乘坐头等舱，你要付的钱比如果不坐头等舱时必须付的钱要多。因此：
To travel first class, you will have to pay more money than is otherwise necessary.
= To travel first class, you will have to pay more money than it is necessary that you should pay if you don't travel ffirst class.
由此可以看出，otherwise含有否定意义，但than从句本身还是肯定意义。即than从句的肯定还是否定不因otherwise的使用而改变。.
现在看网友的句子：
This might sound small, but to undo the effects of such a decrease a candidate would need 30 more GMAT points than would otherwise have been necessary.
=This might sound small, but to undo the effects of such a decrease a candidate would need 30 more GMAT points than it would have been necessary that he should need if it had not been for the effects of such a decrease. .
可见，虽然otherwise还有否定意义，但其表示的是一个否定条件，than从句本身还是一个肯定句而不是否定句。因此不能用否定词替换otherwise，otherwise也并非语法上的否定词。
They support the services that would otherwise be uneconomical.
= They support the services that would be uneconomical without their support.
= If they don't support the services, these services would be uneconomical.
显然，otherwise含有否定条件的意义。
With the teacher's help, I found many errors that would otherwise have been overlooked.
= I found many errors that would have been overlooked without the teacher's help.
= I found many errors that would have been overlooked if the teacher had not helped me.
很明显，otherwise表示的是否定条件。
Well-educated people make more money than they would otherwise do.
= Well-educated people make more money than they would do if you were not well-educated.
otherwise还是if not的意思，表示一个否定条件。
从以上例句可以看出，只要你将otherwise理解为一个否定条件，otherwise修饰的谓语（无论肯定还是否定）为在这个否定条件基础上推理出来的结果，一切问题都消失了。非常简单的一个问题。