Primus 276's page

The question is if shields with attachments i.e. boss/spike can be weapon implements and still benefit from Implement's Empowerment? Implement's Empowerment has the restriction "You don't gain the benefit of implement's empowerment if you are holding anything in either hand other than a single one-handed weapon, other implements, or esoterica, and you must be holding at least one implement to gain the benefit." We know that a shield with a shield boss/spike is a one-handed weapon.

The shield boss/spike has the Attached property, whose first sentence is "An attached weapon must be combined with another piece of gear to be used." Seemingly these two pieces of gear are "combined" to make one piece of gear, as opposed to wielding a shield AND a shield spike/boss. Does that combined piece of gear pass the restriction on Implement's Empowerment, or is this class feature unusable with this type of one-handed weapon?

Page 279 reads, 'Effects based on a weapon’s number of damage dice include only the weapon’s damage die plus any extra dice from a striking rune. They don’t count extra dice from abilities, critical specialization effects, property runes, weapon traits, or the like."
Not counting "extra dice from abilities" seems intended for abilities like Sneak Attack which simply add extra dice for damage (not weapon damage dice).
Power Attack reads "If this Strike hits, you deal an extra die of weapon damage".
The Support ability from a Horse animal companion reads, "Until the start of your next turn, if you moved at least 10 feet on the action before your attack, add a circumstance bonus to damage to that attack equal to twice the number of damage dice."
If Power-Attacking with a +1 Striking Weapon does 3 weapon die of damage, then how much extra damage would be added while benefiting from the Horse support ability of "twice the number of damage dice" (4 or 6)?