Which contract did you use for your purchase? Was it As-Is?
If it is not an AS-IS sale, and you are using the NVAR regional sales contract, the seller is responsible for doing the repairs for termite/wood-destroying insects. So this is not something you would need to be concerned about.
For radon, did you have a radon contingency? If so, you can ask the seller to install a radon mitigation system, which is very effective in bringing the radon levels well below the "danger" line of 4.0.
Your question is cut off regarding the third issue you were mentioning in your question, so I am not able to address it.
Regarding a credit to cover cost of repairs - it is not usually done by % of sales price. It is usually done based on cost of repairs. So you would figure out the cost of the termite treatment (should have been part of the report), and the radon mitigation system (in the range of $2000 from what I understand) and the third issue. All of that added together is the amount of credit or price reduction you should request if the seller is not repairing.
Feel free to call me if you would like to discuss.
I am assuming that you are not being represented by an agent? If you are please address these questions with your real estate agent. Your agent is a huge resource for you and should be able to guide you thru the process. If you do not have an agent mr miller's advise is great. But also feel free to give me a call if you should have any specific questions that I may be able to assist with as I am in your local area. Text or call 703.596.2035.
Also I have include a link to the EPA site about what to do with a positive random test. Hope it is helpful. http://www.epa.gov/radon/pubs/hmbyguid.html.
Why aren't you using a Realtor to protect your rights?
Does that mean that you do not have an Inspection Contingency?
If you don't have an Inspection Contingency you do have any leverage to force the Seller to do anything; and if you try to back out, the Seller can either force you to buy, or keep your Deposit.
What does your Contract say regarding REPAIRS?
Good luck and may God bless