1. Is it an easy mod to remove the factory OEM pipes and install the Borla S-Type exhaust?
its a pretty easy job but if your asking the question, I'm gonna go a head and say its gonna be about 3 out of 10 job. just go slow measure twice cut once.
2. Do I need to upgrade to a Rear Bumper Lip Quad Diffuser Matte Black (such as this one--
https://www.ebay.com/itm/273095941247?ul_noapp=true) as I install the Borla exhaust (so the pipes will fit through the rear)? Also, should I consider another diffuser, or is this a good choice?
I'm pretty sure youhave to get a new diffuser and any quad tip diffuser would be fine, the single is not a big enough hole.
3. Will there be any problem with functionality in upgrading from the current 2-pipe setup to the Borla 4-pipe exhaust, considering the car did not come with NPP setup? Also, do I understand correctly (from another post in this forum) that I can't readily upgrade to the NPP while upgrading to the Borla exhaust?
This I see as your biggest problem as there is no wiring for it so I don't see how you're going to control it other a stand alone switch somewhere.
4. Will there be any effect on power or harmful effects to the motor over the short or long term due to moving to 4 pipes versus the current 2 pipes if I don't get the car performance tuned after the Borla install? Note: I also plan to install a Mishimoto Oil Catch Can and Cold Air Inductions 501-1100-B Cold Air Intake System this week prior to the exhaust upgrade.
Absolutely none, you don't need to tune it at all for doing an axle back exhaust.
5. Any details of which I should be aware that can cause problems in making this upgrade? I'll probably buy the parts and then have the upgrade done when I get new tires (in the next week or so), when the shop has my baby up on the lift anyway.
Research how you plan on operating this before you start buying parts, could get expensive quick. and you can also go the cheaper route of just buying the S-Type single pipe and not change anything out except the axle back. depending on how bad you want valves.
Hope That helps