I have finally found a solution to this massively common problem, and I discovered it by mere chance.
To start off, I, like many users, struggled for weeks and weeks to figure out how to configure my wacom cintiq pen to SAI. The tablet driver property changes didn't work and messing with the shortcut keys didn't either. Not a single forum post, tumbler posting (or even deviantart post) gave a clearly defined answer. I had given up multiple times, resigned to keeping my keyboard near me for the left-handed struggle of pressing the key combos

Until, weeks later, my motivation to find a fix was renewed and at the forefront once more. I knew the answer
had to be somewhere, and I once again made this my personal mission. I had viewed and revisited the various forums and postings so much, google kept my hyperlinks purple.
It was exasperating as I stared into that familiar pit of despair.
Giving up on the internet, I turned to my own wits.
Solution: There under "Others" and in "Options", I clicked on Digitizer Support for the hell of it. I had seen the tab before and had no clue what it was and ignored it. At the bottom, under a section called "button settings" were the options to configure the a tablet pen... And tablet pen...The tail side button is the top one, and tip side is the bottom button.
I just...I was ecstatic. I've come to know SO many people with this exact problem, where other suggestions had failed. I was immediately compelled to make an account on this site right here and now and post this. I'll probably post this to several sites. It wasn't a lonely journey by any means, as I met many kindred spirits whose own questions and postings yielded no results. Thanks for reading, and may this help many more people.
~Antarean
TLDR; Others > Options > Digitizer Support > Button settings (tab)