"I used to be confused, but now I'm not so sure".
I'll jump in and say that reading between the lines, you do not think it a good idea.
Am I right ?
First you have asked for help, a sure sign that all is not well in the decision department. Then you tell us that although there is a certificate [from the Royal no less] you would still think about sending it to the APS !
Then you tell us that your collection is totally MNH and this would be an exception. An exception I think will not sit well in the months or years to come should you purchase.
So my advice is turn down the offer whilst keeping good relations with your friendly dealer and save some of your stamp budget for 'clean copies'.
Is this your most coveted collection or your main one? If it is then my opinion stands strong !
Nick, that's my advice and I'm sticking to it. Good luck and welcome to the SCF.
Londonbus1......knows nothing about Vatican stamps !