No I don’t think that YYJ bearings are stainless. They are steel, but not stainless.
Yes, it fits most YYJ yoyos, but won’t fit on Sunsets, Aqua’s, Bigyos and stuff. It will fit on most though.
Yes, it is .25 x .5 x .187
No, NV is not “GT”, Ninja Vanish is “Flying Trapeze Green Triangle”. Its not just green triangle. Search the full name, you will find that thats the standard name for it. Even the wyyc website says its called “FTGT”.
He sets it up like that because he likes it. Probably for easier thumb grinds and still having a design, i don’t know.