What I really don't like is the idea that someone can combine the Sleeping Tiger with the Synergist Business Coat (they are both from the same manufacturer) for a 16 armor bonus before adding accessories, in a completely corp-friendly costume. It's as good as full body armor and the face has no problem wearing it to the run/meet/going out for donuts/sitting in the coffeeshop.
A single abusive case should result in a slight adjustment to that case, though, such as "this specific case won't stack". It shouldn't be used as argument to make the other combinations, which are far less good, worthless. 13 armor instead of 12 hardly sounds that game-breaking when it costs 6x as much.
As for your better rules, that's kinda silly. SR4 had B vs I and encumbrance on everything, which was the reason you had a billion different combination options for each suit line, so you could tailor your exact ratings to fit best with your encumbrance. Throwing that away and going with an easier system was the right move to make, the R&G stack-rule works much better than an SR4-conversion.
Now you state you feel your interpretation is right, and that's all you or anybody has to back them up. I disagree, which you should know since I already stated exactly how the rules conflict with your interpretation. I feel like you're overfocusing on the + notation, which solely depends on them using 12/+4 instead of 12/4 (which would have caused a different confusion), a single abusive case and a deliberate ignorance of the explicit difference in way things are stated. So you got your feel and the +, I got the actual phrasing of the rule to back me up, so I wouldn't say all we have is feelings as support. Even though I may be wrong, as things are written I actually have the definition to back me up, and all you got is slightly-reasonable doubt on it, and a single abusive case. I'll grant you that RAI isn't clear, but I don't believe you got any solid base for your argument, when literally the only thing you have is the claim that 12/+4 means the same as 12 Armor or +4 Armor Accessory, while the Stack definition doesn't support that claim.
If you want to continue this debate, I strongly suggest you grab the exact definition of how Armor Accessories work, and explain how to unify that definition with the extremely-different-phrased Custom Fit (Stack) rule. I already explained exactly why I don't think the Stack Armors count as accessories, so unless you're willing to fight that by actually going into what the rules say in detail, rather than just the "it's a +" argument, I really wish you wouldn't bother. It really sucks to be reasoning based on what's written and have emotions thrown at you as a counter-argument.