Ok, I have a quick minute to pop on so I'll see if I can explain myself better now.
The fact that your breeder did a harl x harl is not a red flag. I definitely don't agree with them and wish they wouldn't, but sadly that is how most(if not the majority of) harl breeders do it.. and I'm speaking of top show breeders here. Its actually approved of in the Great Dane Club of America's *holy* Code of Ethics.. (gag)
The doubling of the merle gene is *not* what causes deafness in these dogs, its lack of pigment in the inner ear due to the excess amount of white on the head. The fact that this female is lightly marked and hearing is a plus. However, the double merle gene can cause eye problems, but they are not passed on the effected dogs' offspring(at least they haven't found them to be as of yet).
The trouble with a mostly white dane is that you don't always know if they're white because of being a double merle or a merle/harl(studies point towards double harls being lethal and die in utero). A double(homozygous) merle usually looks like what we'd call a merlequin though, so its a good bet that she's not a double merle. However statistically speaking there is a greater chance of her having an entire litter of hearing/sighted harls by breeding this female to a mantle/black, then by someone breeding a harl x harl.
So anyways, I'm sure I made that as clear as mud. I gotta run again, but ask away and I'll try to pop in later again.
TTFN
~moni