Why refer to an advisory circular? Why guess at the meaning of the regulation. Read the regulation; it's very clear.
14 CFR 91.223 provides the requirement that TAWS be installed and operational by March 29, 2005.
91.223(a) specifically applies to airplanes manufactured after March 29, 2002. This regulation requires that any turbine powered airplane manufactured after that date that is configured for six or more passenger seats, be equipped with a TAWS system.
The regulation does not say or imply anything about certification. The number of seats the airplane was certificated with is not relevant to that subparagraph. If the airplane is configured for six seats, and it's turbine powered, then it needs TAWS.
Why quibble or guess weather the regulation speaks of passenger or pilot seats? Very clearly the regulation states "six or more passenger seats, excluding any pilot seat..." Question answered. Six or more passenger seats.
If the airplane is certificated for a maximum passenger configuration of fifteen seats, but utilizes an interior of five seats, then it does not require TAWS. If the airplane is later reconfigured to accept and utilize more seats, then it will require TAWS.
14 CFR 91.223(b) provides for aircraft manufactured on or before March 29, 2002. This subparagraph provides that turbine powered aircraft manufactured prior to that date have until March 29, 2005 to install TAWS if they are configured with six or more passenger seats. It is essentially identical to subparagraph (a), except that these aricraft are given a little more time to get TAWS installed.
If you have a lav seat with no seatbelts, is it still a passenger seat? No, because it does not meet the requirements to serve as a passenger seat. It may be something upon which a passenger could sit, but lacking the ability to have a "seatbelt fastened about the passenger at all times," it doesn't qualify as a passenger seat.
If you elect to modify the airplane by removing the belts, you'll need to do so by way of a Form 337, field approval, STC, or modification in accordance with 91.213, with appropriate maintenance entries, and placarding.