As a point of reference, we don't detect much spam on Software Engineering: see the list here. Some sites get more, some get less, and there isn't much rhyme or reason to it.
At minimum autoflagging accuracy, we would have flagged only one post, from two months ago - this one (link for <10kers).
I'm not sure I understand what you are saying here.
To answer the most obvious question - having multiple accounts that interact by answering each other's questions or cross voting is not allowed and, when discovered, will result in suspension.
However, looking at the question you link to I can't see any evidence of collusion. The accounts of the people ...
There's not a lot we (as moderators) can do other than reject the edits as they appear. Suspending the user would be overkill for one bad edit, however if the behaviour persisted then it might be in order.
A history of bad suggested edits will also result in a ban from suggesting edits so rejection is exactly the right thing to do in these circumstances.
I don't think it is.
Sure it's badly written and probably should be closed, but I don't think its an advertisement.
Wording smells too much promotional, making reader feel more like at an ad board: