Thank you, Brion and Stumble - it all makes sense.
The cleat will definitely NOT hold 18,000 lbs...
Yesterday, I found this in a local rig shop for $75 a piece (2 plates)
It's CSJ-16-210 (stay extender) with dimensions 8.5"x1.5"x3/16". It has a WLL of 5900 lbs according to the manufacturer and about the name number in my calculations for a single plate holding the shackle. The breaking strength of one plate should be around 11,500... At least there is 2:1 safety factor.
Unfortunately, the holes are 5/8". Two of these can be installed with 3 bolts (every other hole), while using the other place in reverse as a backing plate.
What do you think?
For the dyneema vs nylon, I'll email oceanbrake people your suggestions. I think the primary reason for choosing nylon is cost, but will enquire about single braid.
Many thanks!
Gleb