No you're agreeing with my point that if A50 is reversible or not irreversible whichever way you look at it, then it doesn't necessarily alter domestic law, as it could be revoked by Parliament. Thus the Royal Prerogative could be used. 
'Fraid not. IMHO

If A50 is reversible then invoking it doesn't necessarily alter domestic law, so Royal Prerogative can be used to invoke it.
If A50 is irreversible then invoking it does necessarily alter domestic law, so Royal Prerogative can not be used to invoke it.
It's largely irrelevant whether it's reversible or not though because the Govt won't want to go down that route. Doing so is virtually bound to result in a referral to the ECJ which will delay things for 2+ years. Hence the Govt won't do anything that risks a ECJ referral.
To my earlier point that there is nothing in the text of A50 pertaining to whether it is reversible or not. That point is clearly not set in stone as it is open to interpretation. The 2 year negotiation period can be extended with the agreement of the 27, so I think that with the agreement of the 27 a member state could withdraw it's A50 notice. However, given it is uncharted territory a ruling from the ECJ might be necessary.
I meant A50 itself is set in stone - It's part of the treaties and the text ain't going to be modified. I agree that the meaning/intent of A50 pertaining to whether it is reversible or not is unclear, and therefore open to interpretation. The only body that can make that interpretation is the ECJ. The other 27 have no direct say, bar making representations to the ECJ as to why they think it is/is not reversible. I suppose if all 27 stand up in the ECJ and state that they accept it is reversible it may sway the court's decision, but it's a very, very high risk strategy. Anyway, ECJ verdicts typically take 2 years, so a March A50 notification via that route is virtually impossible.