Heads does not protect against Cold Boot attacks. The RAM is also not soldered to the board e.g. with Nitropad X230. However it protects against Evil Maid when shut down as the disk is encrypted and heads offers measured boot. You would notice that someone tampered with it. Som e OS overwrite the RAM during shutdown. Intel ME is also deactivated but depending on the NitroPad, the processors might be vintage.
You can boot from USB stick and also add other OS to heads to boot from. Most Linux should work.
a) Not sure about the purchase / delivery process but I guess the Nitrokey is used to sign the bootfiles so that you don’t need to reinstall when you receive it.
b) Not sure as I use only a single token. But there is an option to add a key. Heads uses regular gpg and the keyring is stored in the firmware. So I guess it allows more keys.
At least with Qubes I can say that suspend to RAM works. Should also be the case with other Linux.
Nitrokey 3 supports cryptenroll via FIDO2. Depending on the OS, this can be used to protect the storage (like an additional password). However heads binds the luks key to the TPM and releases it when measured boot showed everything is fine. After heads boots the kernel, the regular initramfs and OS boot process is used. Thus regular Linux tools work as with a regular system.
However, you can use heads to seal it to the TPM, as @nku writes. Nitrokey advises against it, see “Confirm the question “Do you wish to add a disk encryption to the TPM” with N.” in step 4: System update - Nitrokey Documentation