1.Yes it is. It's not hard at all. Mod ripped, mod posted, money made. It then becomes the mod owner's responsibility to keep an eye out on Steam and make sure it wasn't stolen. If they don't constantly monitor Steam and catch it in the listing time before it's sold, too bad. Valve doesn't do complete refunds and they won't completely remove the mods from Steam barring legal action. It literally happened to one of the very first mods (see: Chesko) put up on Steam's paid Workshop and there are plenty others that are actually straight rips of either mods or promotional pictures.
2. It's not hypothetical when over 2000 mods were taken down from Nexus alone out of fear of them being stolen while others actually were because of Valve's own lack of filtering. It's also causing a very real divide with difficulties caused by mods being interdependent (see: Chesko). Even already existing mods like Midas Magic that were available on Nexus before have been exclusively put on the paid Workshop unless you want literal ads in the game. Implementing this nonsense on Skyrim, an already existing game with a stable modding community, has factually already damaged it.
3. It already happened. Valve does not have a stringent system in place for any of this. This has happened with mods and even promotional pictures, and Valve's official response is only legal action will make them remove stuff completely even in this occurrence (see: Chesko).
4. It already happened. See: Chesko (number 3) and if modders don't catch their stolen mods before they go on sale, Valve refuse to remove the item from Steam completely because of those who already purchased it. Listed mods go through an initial wait period which, according to Valve, "will provide some time for the community to help identify abuse or stolen content and report appropriately. Its also a time that developers can use to review pending items and decide if any intervention is necessary." They're requiring modders to be vigilant in making sure they're mods aren't uploaded against their wishes or too bad.
This is rewarding stolen mods and punishing modders if they now don't constantly peruse the Workshop to make sure their items aren't stolen. Plenty of mods will get through this way, Valve aren't implementing any stringent system of themselves doing anything to ensure this doesn't happen. Once again, Chesko. Already happened with one of the first of, what, 18 mods? They can't even manage that.
The etc. is that this simply cannot be regulated. Valve already have done a horrible job. They're already turning a blind eye to profiting from stolen content and have flat-out stated they will take no action to remove any such content unless someone catches it and informs them or they're given a legal order. The etc. is also causing a strong divide in a tradition of TES modding for no good reason other than Valve/Bethesda's profit. It's also giving modders the raw end of the deal in its cuts. It's doing a lot of things, first and foremost having done an already massive amount of damage.
1., 2., 3., 4. You clearly don't understand how the system works at all. Your arguments are all based on the assumption that all money from sales go directly to the creators wallet which is completely untrue. I am going to follow the popular Reddit route and explain it to you like you were five.
First of all, you need to provide your real legal name and bank account numbers to Valve. This is the first step that makes playing the system considerably more hard than say only needing a PayPal account or a Buttcoin wallet to receive the payments. If you have access to endless amounts of name / bank number combination, congratulations: you are most likely running other scams that are much more profitable than any mod-related scam could ever be.
Second of all, like I said, the payments aren't instant. They are delivered once a month, as you might know if you have ever participated in any Workshop Payment related things. And as you might know with all similar systems, the "getting in" part isn't instant either. So realistically we are looking at a ~3 month grace period before you, me or a scammer could be receiving his first payment.
Third of all, there's a cut threshold of 100 dollars before you get your share of the money. That is that a mod should make at least 400 bucks in direct sales before you would receive any money at all. Combine that with the previous two points, to perform a money making scam with-in the Steam system you should be able to:
1) Provide a real name and bank account combination
2) Make so much money that you most likely would land into the most popular category
3) And you would survive for 3 (or one) months doing that without anyone noticing.
Realistically (or statistically) speaking the mods that pass the threshold are already very popular or will be very popular. Popular things gain attention. And if you are a modder in a "close knit" community such as the Skyrim mod one and you don't notice that your creation is selling tons on the only marketplace around, I don't know what kind of modder you are.
Also Valve doesn't "refuse to remove the item after they go on sale" as you seem to think, they already have and no-one won or lost anything doing that.