By Jove I think you have got it!

Yes, Ebb I think you have got the answer there. I like the shakle pin to the anchor (whatever is the biggest that will fit... 7/16" sounds right. Pin one end of the swivel to the shackle, and in my mind you have achieved the stress relief for the fork/pin on the swivel. Yes, you are right, there is another failure point but in my current system there are 2 shakles and a swivel so I only have one less pin.... and all to use a weaker swivel then your arrangement.

I will look for one of those swivels myself..

Yes! Oh Yes! the seizing wire is a MUST. the more the better (within reason.. unlike the rest of the rode.)