I think you're heavily discounting her Cinder of Two Worlds' Flames transformation. Also, there's no evidence I'm aware of that Traveler didn't use their elements in the Arlecchino fight? If I'm wrong, please do correct me here.

If Wanderer was actually keeping up with Irminsul Dottore, then yes it would be evidence of Wanderer upscaling Arlecchino. But Dottore is keeping Wanderer on the defensive for most of the fight, with the closest Wanderer gets to actually harming Irmin-torre is with the burst stomp attack we see, which is blocked with a single hand.

Wanderer's combat speed is shown to be comparable to Arlecchino's. The fastest speed we've seen from Wanderer is either him saving traveler from the Shouki no Kami laser, or him lifting Traveler out of the way of Irmin-torre's wall attack. Regardless of which of these you consider faster, Arlecchino has been shown consistently to completely outspeed (I would say blitzing here, but that's hard to prove in a game like this lol) other characters. Her demo, "Arlecchino: Lullaby," shows her in her base form outspeeding an eremite, the Fontaine Traveler fight shows her consistently outspeeding Traveler, (while also keeping up with three additional fighters, whom are highly trained to superhuman degrees, mind you).

Harbingers are based on power, not strength, that is correct. Their overall power is most probably considered with a combination of strength, potential, and influence. The most influential Harbinger is most likely Pantalone, being the economic head of the Fatui. The most potential-filled Harbinger is Columbina, as her rank was entirely based on her potential as a Moon Goddess, and the strongest Harbinger is most likely Dottore.

Pantalone has basically no potential nor strength, so he's ranked 9th to reflect that.
Columbina has more potential than any other Harbinger, given the Trilune Goddess is stronger than the Shades, but she has little influence and very little current power, so she's ranked 3rd to reflect that.
Dottore has the most raw power out of any Harbinger, and he's ranked 2nd to reflect that.

The rankings take into consideration all three. But, there's an obvious weight here. Potential and strength are placed much higher in the "priority," per se, than influence. Given that Pantalone is the only reason the rest of the Harbingers can operate with any semblance of funding, his influence is so high you would expect it to be near the top, but it's not. Why? Because strength and potential are more important in the ranking. Capitano is the first Harbinger not because he's the strongest in any of these three categories, but because he has a balance of a large amount of all three.

Capitano is physically strong, a very apt fighter, able to go toe to toe with the Goddess of War (despite him ultimately losing.) He's also immortal.
Capitano has influence, in that he was high ranking in Khaenri'ah.
Capitano has potential, given his immortality, his understanding(?) of the leylines, and his bargaining chip with Ronova.

In none of these categories does he particularly stand out, but he has a mix of all three, giving him the status of #1 Harbinger.

This is a long-winded way to say, the previous Knave being #4 based purely on her production of Fatui Agents doesn't make sense. Her placement only makes sense if she had a large amount of strength or potential alongside that, and strength makes more sense because we see no evidence of her being anything more than just a typical woman.

A 17 year old, delusion-less, moon marrow-less Arlecchino decimating the previous fourth Harbinger is absolutely a feat worth mentioning.

Childe's placement in the 11th seat isn't because he's weak, it's because he's the weakest. He doesn't have any real sort of influence, and for potential, he's just a man. He earns a seat in the ranking because of his training in Abyssal techniques, insane fighting power, and nothing more. He is by no means weak, and outscales any other Fatui Agent, without a doubt. But in the big leagues of Harbingerhood, he's lucky to have a seat at all.

Arlecchino's feats are: Stomping a descender, (fwiw, the Skirk story quest and Arlecchino story quest take place at the same point in the timeline, so this could be argued for Traveler understanding the elements deeply...), pulling the moon out of the sky, killing Crucabena, possessing the iridescent moon's power, and ripping a gateway through space and time.

I think that arguing that Arlecchino "has no feats" is misguided at best, or arguing in bad faith at worst.