Some of this seems really wrong.
The biggest advantage of Horse traders it is that virtually ensures $5 regardless of what happens on T3/4. Get hit with Militia - set aside Htr, draw an extra card, have 5 in hand. Pretty much the only way you miss on 5 is CEEEHtr or the ever present T5 delay of your opening buy. This is extremely important for $5 cards where winning the split will be key (e.g. Hunting parties).
Regarding the value against attacks, ehh I'd say it is strongest against weak attacks. Sure it is brutal against Pillage to completely wiff on the attack, but that normally just means that people avoid Pillage once you've gone Htr or that you play a low odds bet of colliding their attack with the Htr in your hands. In contrast, when Spy is worth getting, it is normally worth getting quite a few so the odds of getting the bonus draw is pretty nice. Htr is likewise good against classic Minion strategies, but I think the huge benefit in Minion games is that Htr converts draw, which is quite cheap with Minion, into coin, which is a bit harder to come by.
Regarding the utility of Htr in engines. Htr, as noted is good at getting your key $5 early (e.g. trashing, gainers, draw, etc.) and it is not a bad way to generate cash either. Say you build a classic engine - it draws the entire deck and aims to buy two Provinces or Colonies a go. A single Htr is fine in a lot of these engines. The final turn before you draw provinces you play Htr as you last card with an empty hand - it is a terminal Gold with a +buy that helped you get a lot of $5 components early. This works with no hitches for things like Merchant Ship, Count, Festival, etc. where you don't need to keep any treasures around.
But even if you have treasures to keep around, you need precisely 2 dead cards to discard. This is quite common. For instance if your trashing comes from Spice Merchant or Counterfeit, the Estates or Shelters will still be in hand and Htr again becomes a terminal gold with +buy. Even with stronger trashing and needing to keep treasures in hand, you still often can work Htr - Chapel, Remake, Lookout, Trading post, etc. often are dead cards late game that you will have to draw anyways; leaving exactly 1 copper around to discard to Htr isn't that bad. Worst case scenario here is that you don't play Htr for a turn, buy exactly one province, and then next turn discard the trasher & the province.
Beyond that, you can have a lot of situations where you can leave junk so that your trasher can hit it the next turn. E.g. I time my copper depletion for things like Junk dealer so that I can Htr away some coppers and then Junk them for another $1. Again you need one turn for this to work before you can just discard provinces.
Beyond all of that, there are many, many engines where draw is low opportunity cost (e.g. Library, Menage, etc.) and Htr works wonders with all of those. Even with more common +cards, part of building engines tends to be making the +draw low opportunity cost (e.g. Tr->Armory can net you a Village/Oracle pair for a net +1 card). Yeah Htr becomes more situational here, but you don't need that much draw to make the discard not hurt too much. But any time you will have a slight amount of overdraw to be green tolerant at all, one Htr is not bad payload.
So where is Htr bad for engines? In the middle build up. You will get hands with Htr before you can draw deck that be Gold/Gold/Htr/draw and you won't be able to hit a sweat $9 and 2 buys, but a lot of boards make up for those by either having a stronger opening (hey I can hit $5 really reliably and don't have to burn a turn one picking up some other +buy later) or by having having cheap draw. It feels terrible to have to leave Htr unplayed in the mid game, but it is much, much better than drawing EECCS on T3 on any board that really cares about $5.
Beyond that there are a lot of places where you really care about managing what is in your hand or discard. Menage is absolutely huge here as Htr can let draw a lot more while generating everything needed for another Menage buy. Library, Jack, and Wt can all benefit for removing stop cards from hand and tossing them in the discard (where you might even draw them at turn end). Tunnel is also nice for using Htr in an engine, buy out something simple like village/smithy, buy two Tunnels, and then power through the Gold. Htr can also be good once you've drawn a hand to ensure that you get the right cards in the discard - e.g. 4 uniques for Harvest, two identical cards for Mystic/Wishing well, cards you want to trash for Doctor/Lookout, making Herald/Golem an assured village, dropping green into the discard for Rebuild (mostly a Necropolis -> Htr -> Rebuild thing), setting up something for Nv (e.g. discard 2 Provinces -> Tr -> Nv). Even something simple like discarding two villages, play a +1 card and setting up for next turn can be very helpful for engine play.
In short, Htr is very good for boards that care about the $5 price point. In extreme cases (like Duke), this is enough to mass spam Htr. In other cases where $5's determine games (like Highway or Rabble), Htr is a solid opening and good engine addition for the end game. It also has a lot of tactical options for when what you discard is important or when draw is low opportunity cost.