No you likely won't be able to put a 3000 series chip in your 1st gen board. OEMs rarely provide the appropriate BIOS updates.
Hmm, you would be trading 4 cores 8 threads for 4 cores 4 threads, but the CPU cores will be faster. For games, yes, maybe a little better. For other tasks, probably a decrease on some things. So if you pick up a cheap B450 motherboard, might be worth it.
GPUs are pretty close in performance, but it kind of depends on which RX570 you have. There will be some titles that favor Nvidia, but the majority should be in favor of the RX570.