Okay, lets try again. I was not trying to respond to your post with an 'iron-clad' defense strategy. You listed a lot of possible contributing factors and asked, "Is there a defense". The answer is Yes.
However, NONE of the specifics you listed provides a specific defense, but in total they could help to mitigate the issue.
For example, lets take a look at each of the issues you raised:
1) "failed to yield at sign on a country road intersection."
Obviously, this shows that he was at fault. This is a BIG Negative.
2) "both gravel."
Gravel should have required a higher degree of safety and stopping distance. Negative.
3) "no alcohol or drugs involved.(drivers tested neg)"
No effect, but could certainly have with different results.
4) "history of serious accident at this intersection."
Could be used to 'redirect' some responsibility to road authorities.
5) "illegal drugs in the other vehicle."
Could be used to 'redirect' some possible responsibility to others (lifestyle, etc.). But of minimal direct value since negative tests.
6) "broad sided the car and killed a 30 year old woman instantly."
BIG negative. Fairly young, lot of life left, possible young family.
7) "visibilty bad due to crops and lighting."
Could be used to help 'explain' traffic situation and possible causal effect.
8) "skid marks from the other car not my son."
Big Negative. Shows that son did nothing to yield.
9) "The other car very old. Salvage titles (several) then rebuilt title. No seatbelts on their side."
Could be used to show that damages (death) may not have occurred if a safe vehicle. Also, that they had SOME responsibility for using an unsafe vehicle.
10) "speed approx. 45-55. The other vehicle 65-70"
Depends on speed limits on EACH road and road conditions.
11) "is there a defense?"
Yes, there are some possible defenses. But remember, my response is based SOLELY on the information you have provided and not on the TOTAL evidence available (to your attorney). As such, you really need to talk with him/her about your issues (above) and let them do what they can.