Conceivably, the couple would prefer a life with a small amount of risk and intercourse to a life with zero risk and abstinence.

Agreed. Whether to take a risk is a separate issue. I simply meant David's example doesn't remove the risk (it lessens it).

We allow some risks in order to optimize happiness.

I agree that we can't cut all risk to zero but in the example David and I were discussing if the couple is abstinent, the risk is zero. It's a case where we can. David was proposing contraception as the solution, the wife is still in danger.

"... is a married woman who is surgically rendered sterile by a hysterectomy (performed for reasons other than sterilization) bound to a life of total abstinence after the surgery?"

No. (This example is exactly what HV 15 addresses.)

"But what about Humanae Vitae15?"
It means you can take a medicine/therapy even if it renders you sterile.

You may never directly participate in a morally illicit action even if it is for good intent.
Say a woman needs to take the pill for medical reasons (good intent).
If the woman is abstinent while on the pill the direct action is medicinal, the indirect action is temporary sterility (this sterility is morally licit under HV15).
If she is not abstinent while on the pill, the direct action is contraceptive (morally illicit), the indirect action is medicinal. In other words, whether she is healthy or not, if she is not abstinent while on the pill, she is contracepting.

I agree with all the points you made.