How do you know that the trial by combat predates Surak’s teachings? I think that Surak’s teachings were not followed by the majority on Vulcan for quite a long time at one point, his teachings had a resurgence in the 22nd century though there had been at least one underground movement that had always continued to follow them. There are also many different Vulcan interpretations of Surak’s teachings, so perhaps many different ways of interpreting their mating rituals too. The Syriannites, for example, were perceived to be an extremist group who were following an interpretation of these teachings which the Vulcan high command at one point found to be unacceptable. In the long term though, I don’t think that the Syriannites ended up being the extremists as their beliefs seemed to be normalised in to Vulcan life by the 23rd century. For example, previous to the 22nd century for a long time the Vulcan’s couldn’t even legally mind meld… it even spread a mind version of an ‘STD’ similar in stigma as HIV has been on Earth. T’Pol even caught this mental disease at one point.
If the Vulcan’s stopped following Surak’s teachings for a few centuries, then why did the Romulan’s leave Vulcan? Vulcan society seemed to be going somewhat in the same direction as to that of the Romulan's as depicted in Enterprise. The Vulcan’s in Enterprise did tend to get quite angry and emotional. Soval for example was often quite cranky. Perhaps the Romulan’s had been influencing Vulcan society secretly over the years and the Syrainnites and other groups like them helped to restore Vulcan back on to the track of Surak? If Vulcan was being influenced in a similar direction to that of the Romulan’s, it would have made unification between the two species much easier from a Romulan perspective, even if it undermined Vulcan and Surak in the process. Maybe the Romulan’s were behind this?
What is logical about committing a murder to find or ‘dispose’ of a mate if you don’t like them? The person being murdered could have been an innocent; a renowned scientist, a starship captain, a philosopher or just a generally nice person with a family and dependents. That loss of life, or any loss of life for that matter, is not logical. No one should have the right to choose who lives or who dies, even indirectly… that would make the person making the decision a monster. It is like corporal punishment almost, but when no crime has actually been committed, or is the crime in the case of trial by combat something as simple as love or even jealousy or anger? Vulcan’s don’t like emotions, so perhaps this was a way of killing people off if they succumbed to their basic and primal emotions?
BTW, did you know that Surak’s Katra is still around? At least it was in the 22nd century. It was transferred in to a Vulcan high priest in an Enterprise episode and may even be hanging around in someone’s head in the 24th century and beyond. Maybe Surak’s teachings had a resurgence as a result of this?
I’m stating all the above as canon when I might be recalling things incorrectly by the way, “it’s been a long time….”
So please feel free to correct any inaccuracies or anything that I have interpreted from canon incorrectly lol. 
