The changed date of Blizzcon has a lot to do with the upcoming D4 Alpha/Beta. I expect this to start right after the virtual event. And from there will be just months to Q4 2021 release.
2022 is too late for D4. From a marketing pov it would be a mistake to delay the game so much. The aRPG genre gets bigger every year and someone may steal the D4 pie out of nowhere if they delay the game for 2 more years from now.
Yes. Trading will probably be one of the last topics since there is a lot of separation on it among them too - marketing guys, devs, security teams, testers etc would all have their own preference on how to do it. Maybe they’ll test the waters with a new trading system coming with D2:R first.