You have a well organized task list / plan.
Re. #5, If the need for a preamp at the C2V was proven, simply lay the system out as follows; C2V Antenna > FM trap (if used) >preamp > preamp power inserter > AC7 > distribution system. The other antenna can be similar; 91XG > FM trap > preamp > preamp power inserter > AC7 > Distribution system.
Re. #6, An FM trap needs to be between the antenna and any amplifier. The job of an FM trap is to protect block ('attenuate' is more precise) FM signals from getting to the input of any and all amplifiers, including preamplifiers at or near the antenna.
Most signal meters are not indicating raw power level, but some form of data error rate or a combination of error rate and signal strength. The individual TV manufacturers don't make the details of their design easy to find or may even consider the information to be a trade secret. Still, I would simply compare the results of single changes... I would avoid adding a preamp and also change tuners / metering, that would leave you to wonder which change is responsible for the altered end result.
I would make notes regarding the performance of the antenna without a preamp... Then add a preamp, (without any other changes) and compare the results. If a strong reliable signal becomes unreliable after adding a preamp, you can safely conclude that either the preamp is overloaded or the tuner is overloaded. Adding attenuation between preamp and tuner will help you understand if the tuner was overloaded or not. Attenuation between the preamp and tuner would be expected to resolve tuner overload, not preamp overload. Attenuation between the antenna and preamp can resolve preamp overload, but leaves you wondering if the the problem was preamp or tuner overload... Attenuation ahead of the preamp affects both the preamp and tuner.