The Mac Pro uses the LGA3647 socket, which I assume will give it some headroom for incremental upgrades in the next few years before a major overhaul is due. Intel already offers 112-thread part for that socket.
No, the 56C/112T Intel Xeon Platinum 9282 is a BGA part, it's not socketed. You can buy this niche product only as part of a server with liquid cooling.
And the successors to Cascade Lake seem to switch to the LGA4189 socket to support 8 memory channels (Cooper Lake and Ice Lake), so I wouldn't expect any upgrades on LGA3647.
