Someone should have received a link to create your account in the welcome email when your support contract was first created. I DM’ed you a link that should work if you do not have that welcome email.
Cumulus Linux is the closest thing to Debian running on a switch. It takes a linux networking config and programs it into the ASIC and it functions similar to a server with dozens of ports that can switch/route traffic at line rate.
With ONIE you can install a NOS, but to take advantage of the switch hardware you would need to write/acquire a NOS that can do this.
With just Debian installed on the switch and only open source software, you might be able to use it as a low powered linux server, but hardware designed to work as a server instead of a switch would perform much better.
Switchdev support would be through a support contract or other agreement with NVIDIA. Sales would be the best place to find out more details. Switchdev support is not something you can simply order on our website, it is different from the way you can easily order a Spectrum based switch with Cumulus Linux and X years of Y level of support.