For the NG categories and knife only categories are the pre order bonus items allowed? The defense coin makes a big difference doing knife runs. Technically you get it in a "new game" but need to have pre ordered I think.

Personally i would say no pre order stuff allowed because its something not everyone has access to but thats just my opinion

That was my thought too. Gonna try knife only without the coin and see how painful it is. All the runs I've seen use it

i'd say no since not everyone pre-ordered

That said, I assume both Defense Scrolls are allowed?

Knife only is supposed to be that, its really knife and healing unless the game requires a weapon use from you. I don't think any NG+ items nor pre-order bonusses should be allowed; that includes defense coin, scrolls, shoes, supply box. The only thing I could say is the equivalent is on madhouse you can unlock the attack and defense coins.

If we're going to allow coins or scrools and whatnot, there should be a NG and NG+ category divide for knife only too.

I agree, I noticed just now that the rules do indeed state New Game only.

Is there a way to get the coin outside of preordering it?

You can get the defence and strength coin on Madhouse, from bird cages on the main hall. Costs like 7 and 8 coins.