Update:

Summary: Three hatches bonded - one complete failure (50% of rim leaking, and I was able to pull the rim off by hand), one with a 2" unbonded section, and the third seems fine.

I used 3M5200 and I thought I'd scuffed up the ABS rims pretty well.
I also did the 'lift the boat by the hatch rim' test.
Perhaps there was some mold release from the rim production, or......

I discovered 'the problem' when playing up at a local lake. Back compartment had about 5" of water in it!

The hatch rim that 'seems OK' now has 12 machine screws holding it to the boat, with the 3M5200 providing the 'gasket'.

The other two hatch rims were removed; the 5200 was cut and scraped from the deck (no bond problem there....ugh); and the deck bonding area was scraped/sanded down to epoxy/glass.
Rims were sanded, wiped with acetone (careful!), and sanded some more when the solvent had completely evaporated.
I drilled 12 holes in each rim, and countersunk the holes (from the 'top').
Bonded to the deck with WEST epoxy with Cabosil and black iron oxide added. The epoxy that squeezed up through the drilled holes was 'flattened' to provide the mechanical backup to the epoxy bond.

It's what I should have done in the first place, I think!
(Sorry, Jay! By the time I read your reply, the 5200 was in place.)