Tips on asking for and getting discount or credit from seller

[Nat C.]

I submitted an offer on a foreclosure at 1K above asking price. The property looked like it required virtually no work.

After my offer was accepted I found out it required substantial work. It has an illegal addition which is impossible to know it's illegal- it appears to be built around the same time the rest of the house was. 

Apparently I was supposed to have been told this before submitting my offer so the bank thinks I made my offer knowing this information.

My buyers agent told me to get a licensed GC to give an estimate for repairs on the house which she will give to the bank to ask for a price reduction. She said the only thing she can see we can request a credit for is the HVAC doesn't work. Apparently we cannot request a price reduction for legalizing the illegal addition as I was supposed to have known about that. I was told it will cost in the realm of $10,000.

Additionally there is an active, severe termite presence which I was quoted $2000 to address. 

I have never asked for a reduced price after an accepted contract before and I don't know the ins and outs. I was told it's highly dependent on which bank it is. The bank in this case is US Bank National Association. 

I know a home inspector (who speaks at the real estate investor club meetings) who claims he gets huge reductions for buyers by finding every tiny problem with the property and making it sound catastrophic in his report. I'm wondering if it would be worthwhile to hire him? Or is the bank unlikely give a reduction regardless?

Anyone with experience in this would be appreciated if you could share your knowledge, as I have none.