Have you discussed this with Comcast?
If you are planning to provide video services to four separate residences under an agreement that covers service one customer, you may be violating Comcast's billing policy. It's been a long time since I worked in the cable TV industry (and even longer since I worked at Comcast), but cable TV companies generally treat each separate residence as a separate "dwelling unit" for billing purposes.
Most cable companies offer "bulk billing" for multi-unit buildings (hotel/motels, retirement facilities, hospitals) billed under a single bill. The per-dwelling-unit cost is lower than the cost would be if each dwelling unit were billed as a separate customer, but the total is higher than it would be for a single customer.
It's certainly possible to connect four dwelling units to a single cable TV drop (as you are now doing) without getting involved in a bulk-bill arrangement. But given the complexity of the project you are proposing, it seems to me that at some point you are going to have to deal with Comcast. I'll leave it to other TD readers to comment on the feasibility of running four internet connections and five VOIP telephone lines over a Comcast drop intended to serve one customer, but I think I can confidently predict that it will be more complicated than simply splitting the cable TV video signal.
As to your specific question... >5) Because of the 4-way splitting of the cable TV signal, we currently > have a powered cable TV amplifier at the point where the current cable > from Comcast enters the house. Will the Internet signals pass through > that amplifier? -- or will they have to be split off and/or bypassed > around it somehow?
It depends on the type of amplifier. If it's a one-way amplifier, then it won't pass upstream signals; you'll have to bypass it or replace it. If it's a two-way amplifier of appropriate noise figure, gain, and bandwidth characteristics, then it should pass data signals. But I can't tell you what the "appropriate" characteristics are: that question hinges on other factors such as the signal levels provided by Comcast, the frequencies Comcast uses for data, and the length and condition of the existing coax inside your building.
My advice: If you are determined to convert everything to Comcast, then contact Comcast, explain what you want to do, and ask them for a quotation. You can expect that it will cost more than you'd pay as a single customer, but the price will include the appropriate design, equipment and service.
As to your question... >
Do you really want to put all of your eggs in one basket? Bear in mind that if Comcast's signal fails, you'd loose everything: video, internet, and telephone.
Keeping your phone service separate provides "route diversity," ensuring that you won't lose everything at once. Notwithstanding the recent sabotage problems in San Jose, I'd keep at least one phone line connected to AT&T.
Neal McLain Retired cable guy Brazoria, Texas