Rereading this, I noticed a problem.
Harry promised Dobby that he would not use blood magic using Dumbledore's blood that he had taken.
The three drops of blood into the lemon drop bag is the probem.
The easy way around it is to get a needle and poke the hanging bit of flesh on Dummydore's ear. Red Cross does it all the time to get the blood needed for a test before a person gives a pint of blood.
Author's response
Heya Disruptor.
You are absolutely right. I blew it. I had Harry use the very method Dumbledore uses, to control him.
What's worse, the use of compulsions is a major thread in the story.
Your suggestion would be good except for one thing. Dumbledore wouldn't volunteer the blood and taking it when he was unconscious, as Dobby did, makes it 'forcibly taken'.
A potential out, is when Harry spoke to Arthur and gave his oath that he wouldn't become a dark wizard, he also said he wouldn't shy away from using magicks that people would consider dark. It's thin, but that's all I can think of.
Once more, you are absolutely right. I screwed up.
Alorkin