Unfortunate that you don't find these threads helpful, there is nearly 10 yrs worth of information compiled in these forums, most questions including "high level" questions have been answered numerous times, if one takes the time to wade through the info freely given. Yes the video are old but mostly still relevant, the trolling industry hasn't changed much, but the players certainly have.. We suggest everyone get educated, as each scenario is different, if you are looking for a magic solution, there is none short of paying the demand. To answer your questions:
1. yup it's a case by case thing, largely depending on who sends the letter, some trolls are more combative and assertive than others, there is always the risk of a lawsuit, the recipient (you) needs to determine the risk level and if you can tolerate 3 yrs of BS harassment in the hopes you don't get served.
2. again they all operate differently, but everyone "pays" something, whether it be money, time & energy, or waiting out the unknown until the statute runs out.
3. nope they don't have to inform you beforehand, that's a common courtesy that most trolls don't possess, they are in it for the money, that is all.. And they haven't sued you yet, they have simply sent you a demand letter..
For whats it worth, I would venture to guess they won't file a lawsuit here, they are only demanding 660.00 which is nearly the cost to file a lawsuit in Federal court, add to that, that I've never heard of cartoon stock, perhaps they are a new player or just not very aggressive..
Here is a thread here regarding cartoon stock, I see they are located in the UK, which brings down the odds of a lawsuit being filed, do you really think they will travel across the pond, hire US lawyers, for a case that would likely not show a profit? doubtful
https://www.extortionletterinfo.com/forum/getty-images-letter-forum/sued-by-cartoon-stock-for-one-image-on-website-but-variation-of-theirs/As stated above each case is different, and only you can decide how to proceed, the more you know the better armed you are.