1. At the outset, you may not like the answer that you will have to turn up to answer this summons, as you have notice of it. Essentially, the obligation on the GArdai is to send the Summons to the last known address of the person or the address given on their car registration or driver's licence. There is no obligation placed on the Gardai to ensure that you receive the Summons. What you do in relation to your own internal postboxes is purely your own personal organisation. A court will not listen to explanations about different postboxes. You are expected to arrange you own affairs so you receive all post coming to the address. So any idea that you will avoid prosecution on the basis of the different postboxes is not a runner in a District Court case for speeding, I regret to say.
2. I regret to say that this is not the law. YOu are entitled to your opinion, but the law is not in your favour on this point.