Form an earlier thread on this subject: list of modifiers.
The mobile bulwark line has some other benefits using a tower shield involving cover. Personally I'd accept that the ACP would be higher than a heavy shield & wouldn't take all of those other feats. What sort of character are you looking at?
Yeah, you'd need more than one feat to make that work. Never mind.
One thing on that list from another thread: there ARE mithral tower shields.
Equalizer Shield "this +1 mithral tower shield".
Force Tower "This +1 arrow deflection ghost touch mithral tower shield".
You have to be able to make the base item, a mithral tower shield, before you can enchant either item.
Also, there is no reason the armor traits don't stack:
Armor Expert and Serpent Runner do the same thing as Sargavan Guard and since they're different categories you could take all three [along with a drawback].
So Mithral -3 and traits -3.
Also being a Tower Shield Specialist fighter helps:
3rd, armor penalty - 3 and Dexterity bonus + 2 and increases every four levels thereafter as per standard armor training.
5th, no –2 penalty on attack rolls.
5th, shield bonus adds to touch AC.
So 5th level with the above is only a -1 ACP and no -2 attack. 7th and there is no ACP.
Lady-J: You really have to be a fighter or throw a pile of feats if you want to use a tower shield well.