in keeping with the side debate on what makes a proper Skybreaker I'd think that the ideal of a "true" Skybreaker would be to uphold the spirit of the law while keeping to it's letter, but that if the choice must be made between the letter and the spirit a Skybreaker will always choose the letter.
I think we can all agree that Szeth hasn't had a chance to truly do either: the law of the Shin states that a Truthless MUST obey his master without hesitation or question, but if Nale's actions can be used as a judge (taken with a grain of salt as he IS insane) Skybreakers were expected to respect the laws of whatever country they are in at the time, so Szeth was put in a "no-win" situation similar to Kaladin's conflicting oaths in WoR, in order to follow the laws of his people he was forced to ignore the laws of the countries he operated in, had he been bonded to a Highspren at the time it would have died.
On the subject of the Highspren themselves having laws against bonding with Humans, this makes a lot of sense and likely explains why no proto-Sybreakers have appeared, however a representative of the Skybreakers will be needed for the Desolation at minimum, so Jasnah, as the Elsecaller and thus designated Spren ambassador, will likely need to go to a Highspren city within Shadesmar to convince whatever lawmakers exist among them to change the law in light of the True Desolation's arrival.
I doubt Szeth will become a true, Nahel-bonded, Skybreaker immediately afterward however, as Nale has been shown to bend the laws he professes to worship for his own purposes (case in point: gaining a sentence of execution against Lift for what amounts to petty theft, a crime that typically would be punishable by imprisonment or mutilation at most in similar societies to Azir) something that a Highspren wouldn't take kindly to. I also believe that due to it's personality Szeth will need to abandon Nightblood before he could become a genuine Skybreaker, as Nightblood would demand that he kill all "evil" regardless of the target's legal, or indeed moral status. In other words, before Szeth can even think about forming a Nahel bond with a Highspren he is going to need to break away from both Nale's and Nightblood's influences, choose of his own free will to put his faith in the very concept of Law and then act on that choice by adhering to both the letter and spirit of the laws of whichever country he finds himself in.
It will by necessity be a long and difficult road, but I have a mental image of a quiet and humble Szeth in colorful Shin robes again, protecting law abiding citizens of all countries and punishing lawbreakers in an appropriate manner to their crime.
Of course he could also end up as my other mental image of Szeth, wielding Nightblood and dressed in white, screaming insane hate at the knight's radiant, and in particular, Kaladin. Only time and Brandon will tell which Szeth will come to pass.
Also, slightly unrelated, but who else thinks that one of the Skybreakers' Ideals is something like "I will dispense judgement in accordance with the crime committed, no more, no less."