Nope, this is the one:
Blizzard Entertainment: Custom Game Acceptable Use Policy
And to answer your questions, some quotes:
This means that whatever mod or custom content you create using Blizzard games is their property, not yours. Thus the answer to these questions:
The answer is YES. They can "takeover" any mod that you make and do whatever they want with it, be it developing, editing, updating or monetizing said mod.
From my understanding this is also valid in regards to any intellectual property tied to your mod, so for example - if your mod spawned a series of books, a standalone game or anything else, Blizzard would have rights to it as well.
From a more practical point of view, such scenario is extremely unlikely - this point is pretty much a safety mechanism in case anything like DOTA ever happens again and let's face it, chances that your mods grow even remotely as big as DOTA are very slim. Thus, unless you are hell bent on trying to use Warcraft 3 to spawn a separate intellectual property (and you actually believe you can manage that), you probably shouldn't be worried as chances that Blizzard will snatch your work are pretty much next to none.
This means that you cannot use any copyrighted material in custom maps, unless you can prove that the copyright owner granted you the rights to use his property.
Now, you can easily find things like music or art that can be used for free in non-profit projects, so you can safely use those. I also believe that anything that people post on the Hive is automatically given such a permission (i.e. the author agrees to grant the right to use his work by other people in non-profit projects), so there's also that.
Other stuff... Well, technically you can't use it without getting permission from the copyright holder first, but - let's face it, Warcraft 3 modding is a pretty niche and small thing, so chances are that even if you use something that you shouldn't, nobody will really notice or care about it do take legal action against you - you'd have to be shit out of luck to have someone not do more than ask you to stop using his content or provide more visible credits.
This obviously doesn't apply to any monetization attempts, but as you will see in a moment, these are generally prohibited anyway.
In short, you cannot make money off of custom games, i.e. you can't monetize your mod in any way. And now, while it's rather unlikely that Blizzard's legal team will come in contact with you due to copyright issues, it's actually quite possible that they might do that over any monetization attempt.
Also, if you read the last sentence, you will notice that you mod cannot contain any features or content that can be unlocked by paying money, so obviously any kind of paid skins are considered illegal.
Here you go - Blizzard gives you permission to receive donations for your work, but... There's some extra rules, such as this one:
What it essentially means is that you cannot give anything for users who decide to donate to you, so once again - no special skins or other rewards, unless you want to risk getting a very "nice" e-mail from Blizzard's legal department.
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So yeah... Generally I feel like the notion of monetizing Warcraft 3 mods is rather stupid. Sorry to burst your bubble here, but let's face it - you're rather unlikely to get much (or even any) money from your mod and trying to do so will only put you at a risk of getting in legal trouble. Personally, I'd advise against it.
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It's not suspicious. It's called marketing - sure, there is no direct profit to be had, but there is an indirect one.
I won't go into much detail here, but the general idea is that if your customer base is happy and well treated, you will eventually get more money out of it. If you're a dick to your customers, they will eventually move on to other products and you will get squat.
Having a nice modding community that creates nice custom games in your product won't upset anyone, but it might make quite a lot of people happy.
Giving modders a hard time won't really make anyone happy, but it might upset a lot of people who like custom content.
A commercial sale is when you someone gives you money to get something in return.
A donation is an act of giving someone money without getting anything in return (bar satisfaction and a "thank you").
Thus, I'd argue that getting a special skin for money is actually a purchase (or very, very close to it), not a donation.
You are allowed to have a Patreon page or provide some sort of a donation link, so that people can send you money if they feel like it.
The difference between this and having a visual perk in-game is that the latter creates a situation where the gameplay experience is different for people who sent you money and for those who didn't. The thing is that some people might consider your mod to be more enjoyable with that skin, thus they might see their experience as inferior to the one of the people who donated. And in order to get that better experience (or unlock it faster), they are required to pay money.
And, since some people would sent you money not to show appreciation for your hard work, but to get that better experience - thus they would pay to get something in return, i.e. they would be making an in-game purchase.
Warcraft 3 is not the only Blizzard game and some of their other games have in-game purchases, so if someone spends like 5$ to get something in a Blizzard game's mod, that's a potential 5$ that he won't spend on Blizzard content. Thus your logic of "Blizzard won't get this money anyway, so why do they care" is false.
However, that's not the primary reason why they are restricting in-game purchases.
If you followed the development of Starcraft II, at some point there an idea to create in-game marketplace, where people would be able to sell maps or in-game custom content. This idea was scrapped due to... legal issues. Allowing third parties to trade for real money in a game creates a lot of legal hurdles, which Blizzard doesn't want to deal with.
Before you say "but Diablo 3 had Real Money Auction House" - yes, it had. And if you followed it's development, you'd know that there were still a lot of legal problems with it, even despite the fact that people were selling in-game content that was generated by Blizzard, not custom made intellectual property with potential use of assets that break copyright laws or suffers from ownership and compensation issues.
To be fair, I totally understand that Blizzard doesn't want to deal with that crap.
