If responding to a recommendation demanded by someone other than the thread-owner, is it advertisement?
If I respond by linking them into a product or just recommend it, then it's automatically an advertisement. If a user wants to get some privacy, I could suggest him to use a mixer and link them to the mixers' list. That wouldn't be an ad, but if I suggested him to use ChipMixer, it's an ad.
If not, what happens if a malicious user creates users posing questions as a method of advertisement?
Questions that have to do with products are advertisements and should be deleted, based on the forum's rules. If it's a question regarding a general topic like gambling, it depends on the replier if it'll be an advertisement or not. Just questioning doesn't harm.
Now, assume that the malicious user is the thread-creator - acting on behalf of a service for the purpose of Q&Advertisement.
Can the thread creator ask about a specific service/platform?
Now that's a great question and I answer it based on the restriction I've understood that is taken from the moderators during my stay here. Yes, they can, because there are numerous times when someone needs to reveal the service. If they're caught to advertise it, though, should get banned.
What happens if an honest platform is given a malicious Q&Advertisement campaign as an associative attack?
The platform may announce that they get advertised maliciously. The mods should act analogously.