Typically go from slab to under side of roof deck, a continuous fire proof layer like sheetrock or densglass on both sides of wall. 

I don't think the ozone machine will work totally until you eliminate the main source of the smell.On the kitchen smell, have you cleaned the underside of the range hood, and taken out the fan filter and ran it through the dishwasher?  

Replace with some nice ABS or PVC, tie it in with some hubless fittings, support the underside, then cover it up.

The fire rated wall must extend to the underside of the roof sheathing.I dont think there is a rated wall that only has fire rated drywall on one side, but there may be some novel assembly that was approved.In my area, to refit the firewall, I had to removal all drywall and replace with 5/8" fire rated drywall, which had to be inspected once it was installed, but not finished (no taping or mudding).

If you did and this was not contained in the report, then you have two possibilities:1) If it is something the building inspector should have caught (you will need your contractor / roofer) to provide you with a professional opinion, then it may be deemed an error/omission by your inspector (he has insurance for this);2) If it can be demonstrated that the seller intentionally camouflaged the defect such that the building inspector could not have been expected to discover the issue [remember the inspector is conducting a visual inspection and is not permitted to open walls, etc. ... though they are expected to enter the attic space and examine the underside of the roof], then you "may" have recourse to the seller.

Where we operate that means using a non combustable barrier (such as dense mineral wool) in the floor cavities with 1 (or 2) layers of 5/8 fire rated (Fire-X) drywall on the underside ceiling.

The drain line is leaking underneath the sink where the rubber gasket gets screwed into the underside of the cultured marble sink/counter.

I stalk up when on sale plus my plumber buddy gives me used but nice ones he's pulled for his higher society customers when they upgrade just because.Sometimes it takes me 12 minutes to change a faucet if the fasten rings are really corroded to the underside of the countertop. 22 minutes if the stop valve is bad and I have to install a new 1/4 turn. 

I am glad I did, the underside was already getting cancerous. 

Without sealing the entire underside of the house the rodents had access to the entire crawl and could make their way into the garage.