I haven't seen any instances of Sam being 'off' besides this Gadreel stuff. I'll have to go back and look.

As for the sigil, I thought that just him smearing the ink or changing it somehow so his fingers end up stained was enough to "add his own changes" as you mention. I figured this was enough to alter the spell and let him gain the upper hand for control of Sam's body. Idk but I can't wait to find out!!!