Harry writes down that he swears that the letter isn't dangerous, but what exactly does that mean? I think you've mixed up a binding magical contract with the spoken magical oath. She can't SEE him make the oath and you didn't say that the oath marked the letter in any way so he basically just promises that the letter won't hurt her when she adds the three drops of blood...
Author's response
Heya Mysteryman, (good movie…silly, but good.)
Technically any contract is an oath, whether written or spoken. Harry was tryng to lay her fears to rest. Think how easy it would be to poison someone via letter. I’ve used this twist before, and incidentally, so did Rowling in GoF. In this case, I used the three drops of blood as a guarantor. If an oath is anchored in blood, it would be more readily acceptable. For nstance, in Kinsfire’s fic ‘Ties of Blood’, Umbridge’s forcing Harry to write I will not tell lies so many times made it into a blood-sealed oath. He literally could not lie. (I guess he’ll have to stay out of politics.)
The difficulty here is the ambiguous nature of magic, and it’s interpretation, both by Rowling and by all the fanauthors, each of which has a different view of how magic really works.
Alorkin