The pins of the SD card need to be facing the PCB of the Pi, skinny end of the card with the pins goes in.
No. The last Model B Pi to have a spring loaded SD slot was either the Pi2 or original Pi3, I can’t remember.
No. HA is designed more of less to run on it’s own machine.
You can run HA using Raspberry Pi OS, but you can not (officially) run a version with the Supervisor as it will not be supported. You can run a Core or Container install of HA on Raspberry Pi OS without issue, but you will lose the Supervisor and all the benefits that comes with that.
See HERE for an excellent explanation of the current installation methods available.
Burn the Raspberry Pi OS disk image directly to the SSD. As long as you have performed the most recent updates to the Pi, it will boot from USB SSD.