I would modify the argument. If we added your suggestion:
(P1’) If there are moral facts, then their basis is either natural or non-natural or supernatural (where these three are construed as mutually exclusive and jointly exhaustive categories)
I tend to agree with E about 80%, where I do agree with him is that semantics is taking too prominent of a role whether one is making the moral argument with categories such as 'natural', 'supernatural', or abstract objects. The real issue is regarding an aspect of our lived experience of the real world. I don't care what you call it, i.e. adding another platonic sub-category as you did shouldn't change utilizing that category within the argument and it doesn't.
So with your change in place I would then change as follows:
(P1) If there are moral facts, then their basis is either natural or non-natural or supernatural (where these three are construed as mutually exclusive categories)
(P2) The basis of moral facts is not natural.
(C1) Therefore if there are moral facts, then their basis is supernatural or non-natural. (I would probably change this to simply a real aspect of our experience of the concrete world regardless of what they are).
(P3) The way we apprehend or perceive moral facts is similar to the way that we apprehend or perceive God.
(C2) Therefore, if there are moral facts, we can apprehend or perceive them.
(C3) If the way we apprehend or perceive moral facts is similar to the way that we apprehend or perceive God then we are in fact perceiving God.
This would (at least initially) avoid the Euthyphro dilemma that E also brought up.
my regards, mikwut
All communication relies, to a noticeable extent on evoking knowledge that we cannot tell, all our knowledge of mental processes, like feelings or conscious intellectual activities, is based on a knowledge which we cannot tell.
-Michael Polanyi
"Why are you afraid, have you still no faith?" Mark 4:40