Congrats on the new drone. Yes, all accessories will work with both drones but the old batteries are a different colour and look odd on the MPP and the old props defeat the object of buying the MPP as will increase noise and decrease flight time. In terms of what accessories are good to get I think extra battery/ies, extra props, good quality case/shoulder bag, powerful 32 or 64gb Micro SD card (SanDisk Extreme Pro is the best) and ND filters. I'd also recommend the power bank adaptor as a great way to discharge your batteries on cancelled flights by using the power to charge up devices in your house hold. And a sun hood. If you end up with 3 or 4 batteries you may also want to invest in the Advanced version of DJI's charging hub. That will fill the case but keep the drone portable. Don't make the mistake of overloading on accessories. If you need to carry your MPP in a backpack then you've missed the point of what this drone is designed for IMO. How long did you have to wait for delivery of your new MPP by the way?

Yes, you can use the same accessories. But buy only low noise props because these are superior. Definitely buy ND filters, some kind of carrying bag, charging hub and gimbal protector with opening I suppose. If your new to drones then prop guards are good accessories to own.

The changes between Mavic Pro and Platinum are the ESCs and props, as well as the color, most of the accessories can be compatible with each other.
For me, I think ND filters and propeller guards are really practical. Hope other users will also help to give some answers for your reference.