You’re right, didn’t notice she would still be alive. Well, Andromeda and Nymphadora were both in the Order of the Phoenix, so them getting Draco would be similar to Dumbledore getting him.

And given that pure-blood families seem to be rather “sparse”, at least since the war, and Lucius is about the hardest of that faction to get rid of, then even if Draco has another living relative or a designated guardian that shouldn’t be a huge obstacle, unless there is a very complex guardianship chain set-up.

So it seems like there should be other ways of dealing with the debt than simply paying it. I’m pretty sure Harry wouldn’t do that unless Lucius does something really stupid, but he probably could do it (using a Dementor is the only idea I have; he probably could think of more). But at least Dumbledore and Quirell should be powerful enough to do it, and might conceivable want to.

For example, if Quirell wanted to separate Harry from Draco and Hermione, and this plot doesn’t achieve that (I’m not sure how Draco will feel about Harry and Hermione now), he could try doing it the other way around by framing Harry for an attack on Lucius: Draco would hate Harry and Hermione could at least stop being his friend, if they’re both convinced. He might even convince Harry that Dumbledore is to blame at the same time. Hmm, I wonder if you could fake Dementation by creative abuse of Obliviation.