Here's a friendly advice and a example of how I've been reporting threads in the wrong section (when I'm unsure where they should be). Let's say a thread was made in Meta.
I report the thread with the comment:" This should not be in Meta (move to appropriate section)". I don't think that I ever got one of those marked as bad. The trick is properly writing out the comment and you're good to go.
Depends on the mod, most will probably mark it as good because you noticed that a thread is in the wrong section. Its hard to tell since I write many reports and my percentage does not change, but I see some threads I reported go to a different section and I think I would notice bad reports.
Correct. I've seen people sometimes report a post stating "off topic" even though it was spam or vice verse (just one of many examples). I usually mark them as good.
Can you get banned or in problems if you report a lot of posts and you are wrong, like having a 10% accuracy with 100 posts reported.
I don't think so, no. I'd say that it would be very hard to get such a low accuracy without intentionally spamming with bad reports. My accuracy was always very high even before I completely understood the forum rules. If you are not good at reporting (under 50%), then just stop doing it. No harm can come from that.