Well now, yes that is close breeding, but there is not reason why the breeding wouldnt take, and there is not really that much higher defect rate than there is out crossing. A lady I knew had a doe that was over 45% inbred, but she took grand champion in all the shows she went to, she was the most well put together doe in her herd.
I have come across parrot mouths and such with out cross breedings, and have yet to come across one with inbreeding. So just as much bad can come of out crossing, than it can with inbreeding.
Technically inbreeding and linebreeding are the same thing. It is called inbreeding when the results are not so great, and it's called linebreeding when the offspring turn out good.
I have a doe now that is about 30% inbred, and she is put together fairly well, her sire is her grand sire.
And yes, breeding them back to their dam or full sister will show any defects.
If it's your only buck, go for it. I do generally try to keep the inbreeding at 6% or less, but sometimes it doesnt work that way.