Talk to your lender and see if they will forward a copy of the inspection report to the appraiser. If the lender has no direct contact with the appraiser, then ask if you can submit the inspection report yourself. If all else fails, you could ask to be present at the appraisal, and hand the a copy of the inspection report to the appraiser.
It will cost you an appraisal fee if you back out. On the other hand, you may secure a lower purchase price if the appraiser chooses to acknowledge the structural problems.
The only time they do anything is if something comes up on appraisal. If you are doing a homepath loan then there is no appraisal. You can try to switch to a diffrent loan that has an appraisal . But they are very strict on this policy.
No seller "has to" anything except agree to the return of your Earnest Money if your contract allows for you to cancel on inspection, and you do so in the required time frame.
The seller considers "will another buyer buy it "as is"? If the answer is yes, then there is no reason at all for any seller to do anything except say "next!".
What happens often is that people try to get to get the home by bidding higher, then get the price they want using the inspection to do so, and ask for a price reduction.
I am not saying that you are doing this, but Fannie Mae typically will rather kill the deal and start over than feel like they were set up for a price reduction.