I'm not sure where you can specifically find a fanfiction with that exact plot. You could try popular fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. Just search for relevant tags like 'Percy', 'Nico', and 'Will' to see if such a story exists.
I don't think it's a very common or popular theme. The Percy Jackson fandom usually focuses more on positive relationships and adventures among the characters. Choking is a rather dark and extreme concept that doesn't really fit the general tone of the fandom most of the time.
One possible fanfiction could be 'The Unseen Pain'. In this story, Percy might be crying due to the overwhelming pressure of his demigod duties. Nico stumbles upon him in a secluded corner of Camp Half - Blood. Nico, initially hesitant, decides to approach Percy and offer his silent support.
Fanfiction.net is also a good place. Just use the search bar and type in key words related to the scenario, like 'nico', 'percy' and 'crying'. There are likely to be some stories there.
Well, in some versions, Percy might be initially shocked. He could be like 'Whoa, I had no idea!'. Then he might start to see Nico in a new light, with more understanding and compassion.
Nico might be initially shocked. He could feel a deep sense of anger towards whoever was responsible for Percy's abuse. Then he would probably become very protective of Percy, vowing to keep him safe from any further harm.
One possible plot development could be Nico seeking justice for Percy. He might start to investigate the abuse on his own and gather evidence against the abuser.
One common plot twist is that Percy actually realizes he has feelings for Nico too, but he's been suppressing them. It's like a sudden realization that hits him when he finds out Nico likes him.
There are quite a few. The Percy Jackson series has a large and passionate fanbase. Some fans are intrigued by the idea of a relationship between Percy and Nico, so they write fanfiction to explore that concept further. These stories can range from sweet and innocent to more complex and angsty, depending on the writer's vision.
There might be some, but it's important to note that such content might not be in line with the original spirit of the characters in some people's views. However, if you're really interested, check out some of the bigger Percy Jackson fanfiction communities online. They often have threads where people recommend different types of fanfics, and you might stumble upon something related to Percy and Nico in that way.
Fanfiction.net is also a great place to look. There are many talented authors who post their Percy and female Nico fanfictions on this site. However, the interface might be a bit old - fashioned compared to AO3, but it still has a lot to offer.