Why is "這麼些" at the end of the sentence, "開會的人來了這麼些" ? Is it a quantity complement?
這麼些 can be directly translated to this many/much, 那麼些 is that many/much, and like it is in English, it doesn't necessarily mean &quot;a lot&quot;:

A: How much pizza did you eat?
B: This much.
A: I can see why you are so slim.

If you do want to say &quot;many&quot;, you want to use 這麼多 or 那麼多.
Back to the original sentence, of course they can be put to the front.
So many people have come to attend the meeting. should be translated to 這麼多人來開會了.
Much (A lot of) work has been done. should be translated to 很多工作已经做了.
By the way, if you use a a quantity complement in Chinese, it usually indicates that you have something else to say:

This meeting only attracts this many people. Is it organized by baboons? //会议只来了这么些人，组织会议的是狒狒吧？
That much work has been done. If you change the plan I'll eat you alive. //工作已经完成了那么多，你要是再改计划我就把你生剥了吃掉。

To sum up, &quot;開會的人來了這麼些&quot; and &quot;工作已经做了那麼些&quot; are not accurate translations. Your book is weird.