Yeah your mileage will vary on the road hazard coverage. For example purposes, we can assume it is forfeited then when you drive away with the tires unmounted.
Total Costco Price if you decide to buy from Costco but NOT have them installed by Costco: $100 Promotion Discount + $75.96 ($18.99 Per Tire Refund * 4) = $175.96
The Defender LTX M/S2 tire I'm considering is $262.99 per tire. Subtracting the Costco promo discount of $25/tire plus $18.99/tire refund brings total to $219/tire. Works out to about 16.7% discount from the list price though, so not terrible either. Tirerack has $70 Mail in Rebate at the moment and offers 2-Year Road Hazard warranty, so that's another option to consider.
Otherwise just go to Discount Tire/America Tire and try and get them to Match Costco pricing and be done with it...
Costco in my experience is very slow compared to most other tire shops, so I generally lean towards using different shop or installer.