This might be a common question but I am definitely confused about what will happen if I purchase a replacement phone from a retailer other than RW, including whether or not I will be able to use it with the same phone number and everything.
As long as the Nexus 6P meets RW’s BYOD requirements you can buy it anywhere. You will need to buy a SIM for it from RW or Amazon. The SIM you buy will be GSM so check the T-Mobile coverage map. If you buy the Nexus 6p from RW and, if T-Mobile coverage is poor where you live, they will send you one with a CDMA (Sprint) SIM.
Whichever path you take, when you activate your new 6P you will be presented with an option to move the number from the phone you are replacing. If that is a legacy model, know that the 6P will go on one of new 3.0 plans, not on the plan the phone you are replacing is on.