Curious about the downvotes, so heres my response:
Depends on the exact method used to DOS. For state-attacks like syn-floods, no, since it just fills up the servers state-tables, but for traffic saturation attacks like DNS amplification attacks, yes, all internal machines would have to fight to get bandwidth from the router.
Once again, depends on the type of attack. for traffic based floods, no, only an upstream entity can do that, but for state based attacks, yes. The technique is called bucket filtering. When correctly configured, they can drop packets into digital oblivion.
Yes, but you would be DOSing yourself, since no one could connect to you. you would have done the attackers job for them. Either way however, you would have to configure legal IP addresses (so you could never use 2.2.2.2.2).
I expect a justification for any downvotes.