I have the 15" version. The screwdriver I have is an iFixit P5...but it seems like Torx T5 is a different type? And I have never heard of tri-point screws. Guess I might need to look into that.
A Torx is like a Pentalobe in that is a star shaped bit, except it is a 6 pointed star (kind of shaped like a Star of David) compared to the 5 star bit of the Pentalobe. It is a common bit.
The tripoint screws are primarily in things like iPhones and Apple Watches, but I believe they are used in MacBook Pros in some places (looks like related to the trackpads at a minimum).
One other thing: If I replace the Apple SSD with one from OWC, does that void Apple's repair guarantee? Meaning they wouldn't repair it if something else went wrong (say for example, needing the aforementioned battery replacement)?
Technically, I don't believe it would unless it is the OWC drive that caused the problem that needs to be repaired (i.e. say the OWC drive caused the failure of the battery or motherboard). Most states require a manufacturer to honor warranties as long as the user only replaced "user replaceable" items. The murky part is whether or not an SSD in a Retina MacBook Pro would be considered "user replaceable". I am not sure what position Apple takes, but odds are good that a court would say it is since the cover is only held in place by screws and the SSD is only held in place with one screw. You might be flack from Apple.
Of course, I would argue you are likely going to replace the battery before you would replace the SSD, unless you want to upgrade the SSD to a larger drive. SSDs will tend to last longer than your typical battery, especially if you are cycling your battery a lot.
And the reality is the a battery is not typically something that is covered under warranty when it comes to normal use. The exception is if you purchased an AppleCare Extended warranty. If so, then my understanding is that Apple will replace the battery if the battery retains less than 80% of their original capacity.
So, unless you want to upgrade the drive, it likely should not be an issue. And if it is, then you can just pay Apple outside of warranty to replace the battery. A replacement battery should be less than $200 (my original MBP that had swappable batteries had replacement batteries that cost $129 if memory serves and that did not require opening up a case and dealing with glue). If it is not covered by a warranty, then Apple should not care what you have done. And if they do, then there are other places to get batteries replaced.