There are two separate issues here:
1. Transferring data plans
This is best dealt with directly through your carrier as CajunLuke notes. For person A, a simple SIM swap should work though, for person B you'll need to cut the 3G's SIM down to micro-SIM size to fit in the iPhone 4. And you can probably swap the SIM from the dumbphone into the 3G and have the appropriate data plans added on the account. Still check with your carrier to avoid headaches.
2. Transferring backups amongst the iPhones
This is pretty straightforward. It sounds like the iPhone users have already got them backed up, so you're most of the way there. Do a final backup, and make sure they remember the passwords.
I'd start with the 4S, since it's a clean phone anyway. When you plug it in to iTunes and activate it, it should ask if you want to set it up as a new iPhone, or restore it from a backup. If it doesn't, just right-click the iPhone in the iTunes source list and choose Restore from Backup. Check to make sure everything seems right on the phone, then you're good to go.
To set up the iPhone 4, start by wiping it clean (this is why you want to do the 4S first, in case something goes wrong, you still have the data on the 4). Go to Settings > General > Reset and select Erase all Content and Settings. You'll have to enter the phone's passcode if you use one. Once the wipe is complete, plug it in to Person B's computer and set it up via iTunes as you did with the 4S, but using the backup from Person B's 3G.
To set up the 3G, repeat the wipe process above, but set it up as a new phone in iTunes for person C. If the dumbphone has Bluetooth sync capability, you can try transferring contacts to the PC that way using iSync and Address Book (note that Lion no longer includes iSync, you'll need a 10.6 machine or lower). Alternatively, if the phone stores the contacts on the SIM, or has the option to copy them there, you can import SIM contacts on the iPhone by going to Mail, Contacts, Calendars in Settings and selecting Import SIM Contacts.
Apple has a good KB article detailing any additional information you might want located here.
The iPhone uses assisted GPS to improve the accuracy of the location system using GPS and cell towers. Wi-Fi networks are used to help determine the device's location; this is called a hybrid positioning system (XPS).
Prior to iOS 4 iOS tapped into the Skyhook Wireless database of Wi-Fi hotspots and their locations. Apple disclosed in April 2010 that they had switched over to their own proprietary database.
If Location Services is on, your device will periodically send the geo-tagged locations of nearby Wi-Fi hotspots and cell towers in an anonymous and encrypted form to Apple, to augment the crowd-sourced database of Wi-Fi hotspot and cell tower locations. In addition, if you are traveling (for example, in a car) and Location Services is on, a GPS-enabled iOS device will also periodically send GPS locations and travel speed information in an anonymous and encrypted form to Apple, to be used for building up a crowd-sourced road traffic database. The crowd-sourced location data gathered by Apple does not personally identify you.
For your security concerns, read this Apple press release.
Best Answer
It's a subjective question and the perceived accuracy of location data may differ based on usage, location and signal availability. While the real answer can only be obtained by actually capturing the data, I'll try to share some information which can help you with your query.
iPhone X along with its contemporaries iPhone 8 and iPhone 8 Plus, are the first iPhones to include support for not one but four geolocation services:
A brief description of each one of them:
GPS: Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Air Force.
GLONASS: Globalnaya navigatsionnaya sputnikovaya sistema, or Global Navigation Satellite System, is a space-based satellite navigation system operating in the radionavigation-satellite service. It provides an alternative to GPS and is the second navigational system in operation with global coverage and of comparable precision.
Galileo: Galileo is the global navigation satellite system (GNSS) that is being created by the European Union (EU) through the European GNSS Agency (GSA). One of the aims of Galileo is to provide an independent high-precision positioning system so European nations do not have to rely on the Russian GLONASS, Chinese BeiDou or US GPS systems, which could be disabled or degraded by their operators at any time. The use of basic (lower-precision) Galileo services will be free and open to everyone. The higher-precision capabilities will be available for paying commercial users. Galileo is intended to provide horizontal and vertical position measurements within 1-metre precision, and better positioning services at higher latitudes than other positioning systems.
QZSS: The Quasi-Zenith Satellite System (QZSS) is a project of the Japanese government for the development of a four-satellite regional time transfer system and a satellite-based augmentation system for the United States operated Global Positioning System (GPS) to be receivable in the Asia-Oceania regions, with a focus on Japan. The goal of QZSS is to provide highly precise and stable positioning services in the Asia-Oceania region, compatible with GPS.
While GPS support has been available since iPhone 3G, GLONASS was added starting with iPhone 4S (ref: List of iOS devices).
Some references from the Web which back the claim that availability of multiple service support will lead to more accurate positioning data: