I'm going to be signing off for the evening, but just a few more things:
I agree with you that the "url" thing in the @font-face is weird. It may in fact not be needed, but EVERY example i could find in any book used it, even for fonts that were just specified as the name, without any web address. But you could try it both ways.
As far as the problem you mentioned about verifying the font and getting the cannot reach error, I do not know. If you browsed to the font file when you did the "add item" and clicked the name, and if that font name appears in the font folder, then I do not understand why it would then give you an error.
I suggest that you look in the Sigil forum on this website, and do a search on fonts. I'm sure there are a lot of threads about the topic. Let us know how you make out!