Gaaah, wasps under the deck, that's tricky.
1. This is a ground-level deck, like right on the ground? No access at all? Otherwise, at dead of night, light it from above with a flashlight and squeeze a long rake under there and pull it off the underdecking, raking it toward you.
2. I don't know about Sevin powder but would be concerned for it moving once it rained, which way does drainage go?
3. Over the door, again after dark, spray it, wait until morning and bag it and take it down. Some people could be injured....a friend got stung on his lip as he was trying to eat a hamburger, that hurts! Otherwise he had no allergic reaction. Hmmm...that's a reminder to
4. Check that you're up to date on your homeowners
5. Grill -- I'd hose out the grill, then scrub using dishsoap, rinse with the hose, drain, and THEN put it on high to get the rest. I'd be worried that merely having it on high will not necessarily get all of the smell gone. But it might be sufficient.
6. Check your gas supplies after (4) above and have a fresh tank ready to go.