It is only with a single weapon that gets multiple attacks, the fire bonus will only apply to its first attack?
No, I think what Endoperez is saying is that you won't get multiple attacks from your normal weapons (the ones that have more than one swing to them, like a sword of swiftness), but you will get the firebless attacks.
Does the physical damage from the weapon itself count as magic?
If a hoof with fire attacks a Black Servant, does the black servant gets its 75% ethereal "saving throw" versus the physical damage and eats the fire damage, or is the hoof as a whole considered a magical weapon?
AFAIK, flaming weapons do not affect ethereal SC's with fire resistance, so probably only 6AP fire damage is magic. I did not make special tests but it always seemed to be so.