Yes, the desktop client functions independently from the phone client once linked (so not like whatsapp that proxies everything through your phone).
> Can I create an account from desktop?
Technically yes, but you either need to compile the desktop client yourself[0] and miss some features or use something like signal-cli[1] to act as the main client. So it's not supported (yet?).
> Is my account independent of any specific device?
The first client that registers acts as the main client, so no. But it might get easier to restore access on a different device without using backups with the secure value recovery[2] stuff they are working on.
> Can I use Signal on multiple computers at the same time with correct credentials?
It's possible to link and use several desktop clients at the same time.
[0] https://github.com/signalapp/Signal-Desktop/blob/development...