I don't hold much faith in hagen test kits, and I haven't had good luck with api
You may get better results from salifert, red sea or elos

I think with 3 seperate test kits all giving my the same results its safe to say the nitrates go from 0 to 15ppm in 7 days.

Another week has gone by with the same results. I would have thought by now that I would at least be seeing a reduction in the amount of nitrate I am reading.

Any other suggestions?

Other than blasting the rock in your tub with a decent powerhead, and rinsing/swishing it really well during your WCs, I think you'll just have to wait it out

Be patient, it took me 7 months to "cook" my live rock from my previous crashed tank. Keep doing your water changes and testing & you will get there.

18 weeks and counting. Still no changes. The rock doesn't appear to bed purging any detrius and nitrates climb to 15 ppm each week. I'm still doing the weekly 100% water changes. Is it possible that I need to do smaller water changes so the right bacteria can balance everything out?

The bacteria live in the rock, not the water column, so you may keep on changing as you were

Would an environment with high nitrates lead to an increase in bacteria that break down nitrates? Is it possible for my rock to be low in that bacteria?

I guess I will keep on and hope that one day the rock is usable.

You're not going to have denitrifying bacteria growing in your rock as they only 'live' in an anerobic environment ( like a deep sand bed )

All you can do is keep removing the nitrates through water changes

Did you buy this LR from another reefer, or was it a new shipment from a LFS ?

I had a sick coral and was battling high nitrates in my tank. So I pulled this rock out along with enough water to start a small tank to house just the coral to see if that was the source. It was the source. I was doing massive water changes to keep nitrates down. Now my tank has zero nitrates, the coral is gone, and I'm trying to save the rock.

If the rock is loaded with garbage, phosphate etc throw it in an acid bath. It will clean the rock spic-n-span however you won't have live rock when your done. In some cases I can see doing an acid bath to have it 100% clean and then seed the rock fresh. You know then what your getting from the rock.