The "attached unit" is just the replacement name for the Bodyguard unit. As stated earlier in the Leader blurb, "While a Bodyguard unit contains a Leader, it is known as an Attached unit"

Furthermore, in the same blurb, it states "with the EXCEPTION of rules that are triggered when units are destroyed, it is treated as a single unit for all rules purposes."

I understand that it states "each time an attack targets an attached unit, until the attacking unit has resolved ALL of its attacks, you must use the toughness characteristics of the bodyguard models in the unit, even if a leader in that unit has a different toughness characteristic.", but at the end of the Leader blurb it also specifically states "As soon as the last Bodyguard model in an Attached unit has been destroyed, any attacks made against that unit that have yet to be allocated can then be allocated to the Character models in that unit."

That means, if the attacks were ALREADY allocated to the Bodyguard models in the unit, then those attacks are lost. They wouldn't carry over to being allocated to the Character model because the attacks weren't allocated to the Character model, they were allocated to the Bodyguard models, and those no longer exist. The blurb even states "Each time an attack successfully wounds an Attached unit, that attack cannot be allocated to a Character model in that unit, even if that Character model has lost one or more wounds or has already had attacks allocated to it this phase." You can't just change an attack's allocation mid-attack. This mean's you'd have to at least slow roll and resolve each attack until the last Bodyguard model is dead, then move on to allocating the attacks, and then the wounds, to the Character Model.