I'd say no, since special rules don't stack unless otherwise stated. Frenzy, for example, doesn't add more aattacks / rerolls on succesful leadership tests, but Dark Elves have some shenanigans that allows them even more attacks if they have frenzy already.
I could be wrong on this, but I disagree. If you have a Str 4, Armor Piercing attack, you get the -1 from Str 4 and the -1 from armor piercing. I don't have a reference for this though.
Two sources of the same ability don't stack. You can't give an elf noble a sword of swift striking and give him "always strikes firster" Same thing applies here. You have armor piercing from one source, and you get it from another source, you still just have armor piercing. Now, if one of the items granting armor piercing is destroyed, you will still have it from the other source. Unfortunately, you are wrong regarding the topic (right otherwise, though). The OP is talking about stacking two different forms of the rule "armor piercing" to reduce armor even further (-2, rather than -1). Strength functions as a separate rule that gives a similar effect with regards to armor. So a strength 5 scar-vet by itself will reduce a 1+ armor to a 3+ armor, while a strength 5 scar-vet with armor piercing will reduce a 1+ armor to a 4+ armor.
And a GW Scar Vet with S7 reduces 1+ AS to 5+, with armour piercing (from, say, a banner) that's now 6+. Strenght is always better than armour piercing, and even Chaos Lords will dread a S7 saurus character, because their 1+ rerollable armour save wont be all that useful. This is why ward saves are so mandatory, because this ignores both strength and armour piercing. +1 rerollable armour save is only good if the enemy doesn't pack insanely high strength attacks. though if you want to go full tank, 1+ rerollable with 4+ ward save will make you are tough nut to crack for pretty much everything. Too bad you can't have that along with T6.