You should be able to connect Apollo to the H9000 via Word Clock without the T-piece terminator. I have connected these devices in this way without any issues. However, if you have always connected them in this manner with the T-piece without any issues, you can leave them as is.
Yes, I would recommend clocking the devices via Word Clock when using an aggregate device. Otherwise, you are leaving the clocking entirely up to the aggregate device, and some users find this unreliable with any devices, not just the H9000. Either device can work as the clock source, so having the H9000 follow the Apollo is not a problem. In the Audio Midi setup page, you should uncheck the “drift correction” box for all devices, since this is not necessary when you are physically clocking the devices using Word Clock.
There shouldn’t be any difference between using an aggregate device with an M1 machine or a machine running a newer OS, so I’m not sure why you would now be having issues if you didn’t before. We’ve tested the H9000 USB driver with macOS 12 and M1 machines and have not experienced anything strange, but when using an aggregate device there is always a chance of something not working reliably.
Usually if a USB hub does not work with an H9000, the H9000 USB device will simply not show up, but it’s possible that this could be a factor in the issue so you could try a different method of connecting the H9000 to your computer.
Let me know if that helps.