First Time Rooter (primarily for tethering), keeping Android Pay

I have never rooted an Android Phone. I have a new 6P, and have lost the ability to tether that I previously had on my Moto X 2014. I understand that I need to enable it in the build.prop file (and to do that, I need to root). However, I am also a regular user of Android Pay., and don't want to lose it.

Having never rooted before, I am looking for the easiest to follow/most foolproof method to root so I can enable tethering without losing Android Pay. I am fairly technical, and have previously jailbroken by iphones. But, since I've never rooted Android, I'm looking for the clearest/most direct path. For the time being, I don't have requirements for other functionality (via rooting) beyond Tethering, however I will begin to explore the rest of the rooting/ROM world after it's working smoothly with Android Pay.

I would appreciate a pointer to the set of steps (as "cookbook" as possible) either on this site or elsewhere that will give the greatest chance of success to a first timer. (From what I read, may have to be a "systemless" root in orderto keep Android pay).

Any and all assists/pointers/comments are appreciated!
P.S. If it makes any difference, I'll be using a Mac for the root.

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Please help. Just bought a new S5 & have a question about enabling native tethering

I just purchased from the daily steals special unlocked galaxy S5. Being that I am from Canada, and because the description of the phone was in error I did not realize I was purchasingThe AT&T version which is locked down. While I don't strongly feel the need to root the phone and customize it, I would like to still have full functionality which includes native tethering which on my plan, with my provider in Canada I am entitled to. My understanding is that this version of the phone has the tethering locked down to AT&T. Being that I have not taken ownership of the phone yet I do not know which firmware is installed on it to see whether or not the option of rooting the device on kitkat (and changing the tethering can be done) and then bringing firmware up to lollipop is an option. This is something important to me as I use this feature often.
I would like to ask the following questions:

1. Firstly I would like to know when phones are purchased new do they always have KitKat firmware on them for this model or were any of you who purchased the phone did it come with lollipop out-of-the-box? This is supposed to be a new unlocked model for sale. It is not a refurbished model.

2. Question is there a way to know based on the serial number on the outside of the box what the firmware is? The reason I am asking is because when I pointed out the error of the advertised description of the phone they told me I am entitled to return the phone without restocking fee if I have not opened it yet. That it is already on an unrootable firmware.

3. If by chance this phone is on lollipop, is it still possible to enable native tethering if I am unable to root the phone? I have seen mention on an earlier thread where people used ADB and were able to pull the tethering provision file modify it and push it back to the system which would then allow native tethering to work. I am just not sure if this was specific to pre-lollipop firmware. I have also seen somebody advertise on YouTube it he for a fee is providing the service to enable native tethering on the galaxy S5. However he is the only person I have seen mention this anywhere so I am not sure whether or not this is something that can be done without root. Does anybody have any experience having done this?

I am unfortunately in the awkward situation of having this phone currently be in the US and be brought back to me in the next few days. So I essentially have the weekend to make a decision as to what I should do. If anybody can provide me any insight I would greatly appreciated.

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