Should we use eugenic reasons forbid the marriage of women over 40 planing on having babies as well?

Yes. We should also encourage the use of sperm donors as resulting in 1/5 the birth defect rate. (On a large scale, this advantage would erode, but that is easily dealt with by encouraging men to donate more and treating them less like crap; we can think of it as 'sperm banking', akin to existing practice of 'egg banking'.)

In the Western world most people are pretty out-bred, one generation of inbreeding does very little damage

Do you know this, or are you guessing? If I were to go looking for any studies in out-bred populations, at how many points of average IQ damage would you concede the point?