Hi and welcome,
Another take on your questions.
Should I purchase two Obi 110's to accomplish what we need?That would be ideal as each OBi can be setup with 2 GV accounts. Since you plan to have 3 GV users it sounds that you do not want to share a GV account. Two OBi's is the way to go in that case. Note that you can make 2 outgoing calls with a GV account at the same time.
Can we all call out separately without having to input the **?Yes. With the proper dial plan setup that would be possible. You could setup the OBi box with the "Trunk" setup as the primary line. Each OBi can be configured to line hunt in this case allowing you to make a direct call without pressing any extra digits. A few things can be setup to give you a regular phone experience.
What are most people using for 911 service?I think some people rely on their cell phones for 911. This service is free on a cell phone, not requiring a monthly package or prepaid plans. If you have one or can get your hands on a used one, it could do the job. Others with voip accounts rely on e911. You could Google it for more of a in depth explanation.
You could do what jimates is suggesting. Every 911, 311, 411 etc... has a regular phone number you can call. All these numbers can be configured into the OBi unit, giving you "normal" expected telephone behavior.
Read this thread I explain how it can be setup. The general concept works some tweaking might be involved depending on how you want to configure everything together.
http://www.obitalk.com/forum/index.php?topic=124.msg386#msg386Hope that helps.