You can create a new user on switch to get a new save slot so that you don't need to delete your save where you have 100% the DLC. Locking the DLC-items means to "lock" them in the weapons list in shop, smith or equipt screans by pressing some buttom whule hovering over them. Locking stops you from selling or fusing the weapon by mistake. You aren't required to do it but it is recommended so that you don't invalide your run by mistake. Only way to fully disable the DLC would be to factory reset your switch and then not download the DLC. Because of that if you want to do the NG run using DLC-items go for it, we in the mod team will then make sure there is a valid category for it. If you decide to do the run under current rules, we in mod team probably will compensate sometime for DLC related time loss.