1) I can not personally attest to the link you provided. When I provided the inssider link, I wasn't aware they are now charging for it. Older versions used to be free for personal use. There are similar program floating around that are free if you do a little searching.
2) Yes, you can update the password. You have full access to the router. Make sure you buy a VZ branded router. There are other versions of the MI424-WR floating around (Actiontec generic, Frontier) that may have different firmware.
4) No, you do not have to turn off the primary router when following the FAQ. Just be sure the secondary router is disconnected from the coax when instructed to do so,