Naked Necks are indeed their own breed. However, typical hatchery stock Naked Necks are more just multicolor, dual purpose birds with naked necks than true Naked Necks.
The naked neck gene is dominant. So only one parent is required to create a naked necked chick. A bow tie indicates a heterozygote whereas a bird with a fully naked neck or very small bow tie indicates a homozygote.
That depends on what you mean by "huge NN." Do you mean a bird which fits the standard breed description for a Naked Neck but is much larger? That would require a strict breeding program and many generations of breeding to achieve. However, if you simply mean a very large bird who also has a naked neck, then yes, a simple cross to an Orpington or other large bird would be a start to producing that. Size can be tricky to breed for, though, so it may take more than just one generation to get a really "big" bird. And of course all F1 offspring would be heterozygotes for the NN gene, and so would have bow ties.
No breed is simply "sex linked." Some breeds are Autosexing but these are rare. Sex links come from a specific cross of colors, not necessarily any specific breed. You could cross two mutts and get a sex link, if they so happened to have the correct genetics for it. So, if you were to breed a Black or Red Naked Neck cock to a Barred* Naked Neck hen, then yes, the offspring would be sex linked. This cross would work in any breed or cross, Naked Neck or not - for example crossing a Red NN cock over a Barred Rock hen would also give you sex linked offspring.
*I have no idea if Barred NNs are a currently extant variety. I was using a "for instance" example.
Showgirls are the product of many generations of breeding. It would take many generations to go from a F1 NN X Silkie cross to a real Showgirl. Even many of those who have been working on them for many years have still not achieved "perfect" Showgirls. F1 birds will be bow-tied, most 5 toed, with partial crests and beards. Melanization of skin will vary. None will have Silkied plumage but all will carry the gene for it and so can produce Silkied offspring if bred to each other or back to a Silkie.
If you are interested in poultry genetics, I would recommend reading
this website (if you haven't already) and taking some time to play around with
this genetics calculator made by the same guy who runs the website. Both are very educational. I'd also recommend picking up a copy of Genetics of the Fowl by F.B. Hutt if you get the chance.