MicrostockGroup

I don't think so, as they don't have RF editorial.

Nope they're very clear that a teeny tiny person or a part of a person counts as a person.
"Why you need releases for people and property
    To maximise the potential to sell your images for “Commercial use”.
    To sell your images Royalty Free.
    If a person can recognise themselves in an image. Examples when you need a model release also include, crowd scenes, team sports, and scenarios when the face is not visible such as parts of the body, or silhouettes. For a picture of two people shaking hands, where only the hands are in shot, you need two model releases."
Source: http://www.alamy.com/contributor/help/image-releases.asp

You can watch the videos on their site where  a contributor on a conference asked the same question and the CEO clearly states that it doesn't matter if the people are recognizable or not under NO circumstances they will accept pictures with people as RF if the are not released - watch it!

I'll try to answer from my personal point of view. Stock agencies make their own rules and what's legal isn't always what they require or demand.

No legally you may not need a release and the buyer probably doesn't need one to use it, but the selling site requires it.

Some other site may not require it and you can sell the same image RF, but Alamy does.

This also carries over to other areas, where SS takes an image as Editorial and it sells well, IS refuses it because they don't think it meets their editorial standards. I can upload a PD image from the 1800s to one site, while IS refuses it as "needs a model release". 

In the same way, an unidentified person on one site, blurred, from behind, or so tiny, that they could never know it was them-self, is fine and suitable for RF. Alamy says, no it's RM or you need a release.

I wish I could find the post where someone had shot a golfer from behind, at a driving range I believe, and the person in the picture (claiming to be) has filed a suit with the agency. After some time and trying to get money, it finally got down to the fact that the picture was shot in a different country and it couldn't have been the person making the claim. Now somewhere the agency had to defend that and spend money and the artist had to prove that they were never in "X" and prove where they did take the picture. PITA for everyone because someone wants money for being shown from behind in a photo?

We never know how many people will come out of the woodwork saying, that's me, if it's a big ad campaign use?

So they error on the side of caution. This is one of the problems with Micro or any other stock. CYA policies.

You certainly would on iStock:
http://www.istockphoto.com/forum_messages.php?threadid=68478&page=1
at least in theory, though I know that isn't always followed.

Not my case and the person who was accused would need to fill in details. My only point was, people will and can make claims, without substance and it costs money to defend.

As for your second question. No the cost isn't always assumed by the claimant, especially if there is no trial and case, but it just was someone going to the agency and threatening them, until the details were revealed. That's why the whole thing is a mess.

When you write to someone and say they are using your image illegally (hypothetical situation of course) and it turns out they paid for it. Do they sand you a bill for the time it took to look up and prove they had a license.

See where this all leads? If there's a proper release, the issue is ended before it starts.