The Particle team is working hard to fill all those backorders, and I'm sympathetic to the reasons they are delayed. You won't hear me complaining about that. Its good to hear an acknowledgement that the customer facing order software needs to be replaced. And for sure, right now is not the time.
Well... Order mixed with Preorders can be handled gracefully. iTunes, Barnes and Noble, REI, to name a few that I've used for pre-orders and orders from stock. When I check my order status, it clearly shows what is on the original order and what has been shipped and what will be shipped, and a projected date. In the case of iTunes, it is pre-released music, but still, its the same.
If you want someone to research platforms for you, I'd be glad to help out.