Rigging hardware: sealing core and bedding compound

I don't believe so, water will find it's way in there and could delaminate between the waterproof core and epoxy resin given enough time.

I totaly agree with the A/M responsesI have only one thing to add
Better safe then sorry !!!!
And specially at sea !!!

You are combining two issues into a single question.

Bedding: obviously you want to bed any hardware (cleats, chocks,...) with sealant to prevent water from getting inside the boat. For hardware mounted outside, where water woudl just flow into the cockpit, it is still good practice to do so.

Sealing the core is another issue altogether. It is critical whe using balsa as a core because it will suck any water and rot. Over the years rot will expand away from the original hole resulting in massive delamination. In theory, the sealant should prevent water from coming in but with time it will happen. So the correct way to do it is to drill larger holes, fill with thickene epoxy then drill thru the epoxy blue to install the bolts.

Now when using closed cell foam core like Divinycel and others, you don't really need to this, you can drill thru and rely on the sealant. Over trim should a little bit of water get in, it can't rot the foam and the closed cell will no absorb water

So yes, with foam core you don't need to seal the core but you need to bed the fittings with sealant.

5200 isn't the best choice as it s harder to remove... 4200 UV is a good choice.

Do you have to, not if it's actually a closed cell/epoxy construction but it's good practice to bed/caulk anyway. If the boat ever gets to freezing climates moisture between the core and skin will expand and begin delaminating. Many times when installing screws the skin will get tugged away from the core in a small area and moisture will get under. Exposed core is not a good thing in any case. Most of the foams have pretty poor peel strength and an exposed edge is begging for delam to start. Even "minor" core penetrations that are "just" holes through the core like anchor rode pass-throughs should have the hole protected by backfilling as Pascal described (best) , glassing (next best) or coated with neat epoxy (bare minimum).

Bedding the fittings is cheap insurance.

This also prevents the compression loads created by the fasteners from damaging the laminate. All compression will be taken by the hardened epoxy instead of deforming the core and weakening the structure.

Screws ripping out???

Any harware csrrying any load like cleats, chainplates, chocks, outriggers, etc should be THRU BOLTED WITH BACKING PLATES.

Screws are for trim bits

If the "builders" mucked this, I hate to think about the other things you can see..

You may want to consider hiring a surveyor and attempting to recover some $ from the builder, there is no excuse for this kind of stuff...

The same little chemistry game that water plays in an osmotic blister WILL take place in the capillary cavities of ANY penetration of the laminate even with foam. Balsa just rots faster, the osmotic process never gets a chance. Bedding, and thus keeping the water out, is CRUCIAL in ALL laminates. Even better, drill, remove the coring well past the hole ( I like the bent nail technique) fill with epoxy and then re-drill for the fastener, and then still use bedding compound. Do this for EVERY hole.

You have just become the poster child for the theory that even a new boat needs a pre-delivery survey.