I'm having a hard time understanding why any of this matters. Why do you care? Is it for your own filing purposes or are you considering the audience??
Tags do two things.
The help you organize the things you write and the threads you want to read. I tagged one of my threads spamalot. There is no spamalot forum per se, but if someone searches spamalot, they might find it. I don't care if anyone searches for the thread or not, or if they use the spamalot tag or not. It's their choice.
Forums are general catchalls. If you want to align your topics to a forum, GREAT. Awesome. If you don't, c'est la vie.
If you write a thread on cooking, sure put tag it as a cooking thread, or don't. Chances are some one will anyway. Somebody else might tag it Chinese food, or snuflufageous. In the end, if it means something to someone, they'll tag is how they see fit or they wont. It's only important if you want it to be. I hardly tag anything. If I want to find something, I'll either use the search button or look in forums or ask, sometimes other people, esp. Butterflynet have great memories and can find anything.
As for Discussions and Questions and the difference... One of them has the red ribbon, the other doesn't.
Tags are linked from your profile page.
Again, you are WAAAAAY over thinking this. Relax.