I didn't know there was actually two lethal alleles. Is that rare in one species? Hhhmmmm...I wonder.
No, it's not rare. There are many, many known examples of lethal factors, and most likely many more than that that we don't know about.
So, if reabsorbing a fetus is not detrimental--is there actually an issue with breeding parents who might have homozygous mutant allele babies? Or is there a risk of actual abortion too?
Whether or not it is detrimental in the long term is not known. I'm not sure what you mean by actual abortion - the embryo does abort in the biological definition of the word: cessation of development/growth. If you mean abortion in the sense of an expelled fetus, no, that doesn't happen.
The issue, in my opinion, is quality. I am not as against black x black pairings (when done by those who know what they're doing, and not by BYBs solely to produce fewer standards) because blacks are the overall strongest mutation, almost on par with standards. If the parents compliment each other, there's no reason why they couldn't have offspring that are an improvement on them. However, I personally still have an issue with putting a female in a position to reabsorb multiple kits over her lifetime, and as you can still get quality black offspring breeding a black to a pure standard I just feel that that's a safer breeding practice for the female.
White x white pairings, however, I doubt any offspring would be an improvement. The major faults in whites tend to be the same: cottony fur and yellow cast. Doubling that with another white parent would just produce weaker, yellower fur. Show me a white that compliments another white better than every pure standard out there and I will change my mind on this.