Girardi

It's a scam, in much the same way as the 'We can help you claim that PPI you're entitled to'.
 
Re claims, when a claim comes in there is usually a 'reserve' put against the claim based on the expected total cost of the claim. As the claim is handled and more information becomes known or payments are made to accident repairers or loss adjusters, this reserve is increased or decreased accordingly. It's a regulatory thing - the company has to provide proof of funds to cover the claims they have on at any one time plus potential claims likely to come in, a bit like a safeguard.
 
Once all payments are made and the claim is closed off, the payments are considered against the reserve and the reserve nets out to zero. So yes, some money is probably 'reserved' against the likelihood of a potential injury claim, but if your claim has already been processed the claim would have to be re-opened and re-reserved. There isn't just a little pot of money sat there waiting for a fraudulent claim to come in.
 
These companies get your details by either buying lists of information from people (you only have to forget to click one of those 'don't share my details' boxes on a form somewhere and you're essentially fscked forever) or by using auto diallers on a phone where they don't know anything they just dial a random number and you happen to pick it up. There are whole companies dedicated to purchasing and collating databases from online retailers, grocers, energy suppliers, double glazing salesmen etc etc and combining those together to sell on to marketing companies as 'cold leads'. Impossible to know how they have obtained your information, but you will have inadvertently consented to this somewhere along the lines.
 
Re question 3 - the scam companies themselves take no money from the insurers - the insurers probably don't want them to exist. More (potentially fraudulent) claims = higher claims costs = less profit = angry insurer with less money.
 
And on the other stuff... yeah, there are entire teams of people looking into fraud and as an industry insurance is quite hot on all this because of what I said above. it always looks fishy when someone 'remembers' that they had un provable whiplash in an incident from 2 years ago.
 
Bottom line? The companies that phone you about these claims are trying to get you to do something so they will get money. If you really think you have a genuine claim for a genuine injury that genuinely happened, it really is as simple as phoning the insurers...