About your first question, it's straight-through, no doubt.
As for the rest, I'm not too sure, but would love to know.
Here is an extract from thw www.surfboard.com web site (my cable modem is a SurfBoard):
"I have more than one computer. Do I need more than one Motorola SURFboard computer modem?
No, if your computers are connected via a network, one Motorola cable modem will support up to 32 computers. Since each computer must have its own unique IP address - which you can order through your cable provider or ISP - expect to pay a little extra each month for each additional computer you tie in. "
Tell me what's that supposed to mean!!