B) You do not need an Obi to do this. GV can and does forward to a cell number without the Obi device in the middle, it can ring/forward to up to five numbers simultaneously. Some of your questions are asked in a way that appear you may not know this already.
1) Definitely yes use separate accounts for GV, separate from any "main" gmail account. YOu can always forward any incoming messages to the main Gmail account.
2) Yes. two additional separate accounts.
3) Common dilemma. No one answer fits everyone, because of the various tradeoffs.
One possible approach would be to get a GV #, give it out to a few people and determine how your usage works, what is important, to gradually wean people off your normal cell #.
There's also the big "Number porting to GV - my experience" thread on here. It's mostly people who port their land line to GV, lots of insightful stuff on there.
4) see answer B above. An Obi100 (or 110, or 202), while it can forward incomign calls on each SP (Service Provider, in case SP1 and SP2 will correspond to GV #1 and GV #2) to a different number, it's highest and best use is ringing analog phones attached to it's physical ports. Picture it this way: In a traditional analog phone arrangement, say with Verizon Landline, the phone wires that connect your house to verizon are "cut off," and replaced by the Obi device. One or more of the phone ports on the Obi device give you the dial tone that you previously got and paid for from Verizon.
5) An Obi 100 can handle 2 GV accounts, and one analog phone line. You can use this. An Obi110 or Obi202 add a few more Gv accounts or analog phone lines.
6) Switch or router either will work. It's works just like at any other network device.
7) Yes, I'm not sure what page plus is or does. Best course of action if you're new to all this is get the basics working first.