It is not. Specifically SealCoat is a dewaxed shellac. The Varathane is a Water based Acrilic resin. Seal Coat is the only product of it's kind that i know of. Most often dewaxed shellac is sold as flakes that the user then dissolves in ethyl alcohol.
Shellac as stated above is a magic product that sticks to literally everything. And everything sticks to it. (not literally there may be some stuff that it doesn't stick to or vise versa)
Unfortunately i don't know if Home Depot carries the product. I buy it at Menards and don't really know all the places that it can be found but i linked a store locator below you'll have to check each specifically for SealCoat.
No its not the same. Sanding sealer is a product used to make a smoother finish in open pore woods. The Seal Coat is a shellac product, the shellac being the magic ingredient for getting dissimilar finishes to stick to each other.