No, you can't. The maximum "dollars off" promo code was changed to $50.00 a while back.
Yes. They are managed by way of account "credits" for registrations of domains with DreamHost as the registrar. Obviously, any domains to be registered using the credits will need to be registered at, renewed at, or transferred to DreamHost (as they are not offering to pay for registrations at other registrars). Note that this, by necessity, is only applicable for TLDs DreamHost registers.
I have no idea what you mean by this question. "Hosts" and "users" don't have much meaning in the sense of registration. Any "users" on the account granted the promo benefits will be able to use them (if they are granted those privileges in the Control Panel by the account owner). Once registered, the domains can be "hosted" anywhere you can want presuming you point the DNS to the other host.