As Kazen said, TH definitely works like a debuff rather than just being on the hate list. You have to directly act on the enemy. However one action should apply the highest level of TH you have equipped. There also seems to be a range aspect to it. If a monster wanders too far from the THF, like in Dynamis, they seem to lose their TH effect (not sure what the distance is -- maybe 20 units?).

It's definitely important to make sure the THF has acted on the mob recently (in case the mob has wandered off or the THF has died, for example) and is still within range of it when it dies.

There are still some superstitions that people claim to believe on Nasomi, such as THF kill and whatnot but I don't believe any of that.

Also does feint work and is it exploiting to use it for TH. Like if you wait to use it until end of the fight because tool tip says adds to TH.

I had a lot of luck applying feint to mobs before they die when it first got fixed and I subscribed to the theory for a long time. After doing it enough times with and without, I don't truly think it does anything. I do still do it though and often joke with my ls thfs coming to tag stuff that they should too because a mechanic such as treasure hunter is always going to leave us with a degree of superstitions and confirmation bias's so when someone says why? The answer is often "Well why not?"

As a blu who regularly /thf on hnm and such, I strongly believe this to be the case. Coincidence, superstition or truth, I err on the side of caution

If this is the case, I wonder if TH affects the drop rate if the thief 1-shots a mob. Anecdotal, but whenever I kill something in one hit, I tend to get fewer drops. I don't have anywhere near enough tests or examples to prove this, however.