I would think if it is one of the fish doing this that they would not only be doing it at night. My best guess is there is a disease doing he killing and scavenging doing the rest. There are any number of things that might be causing death without obvious symptoms. Columnaris is not a single strain of disease, it comes in a variety of strains. The worst can kill fast with no warning signs or symptoms beyond the fish died suddenly. Then there are the least aggressive strains which can persist in a fish for months without killing it.
There can be parasites as well which can be fatal and remain hard to detect. And this is one of our greatest problems in he hobby. We can tell when a pet cat or dog as well as many other animals kept as pets are not "right." We usually see sym[toms. But with our fish this is much more difficult. And even when we see the outcome or that fish are indicating that something is amiss, we will not have any clue what exactly is causing the problem.
I have to lean towards it being a disease most likely. The fish you have are not known as killers save maybe the rams when in spawning mode. But they defend their young/spawning site and do not go out hunting. All of the fish are likely to sleep at night save maybe the pleco. It is a bottom dwellers and i doubt all your fish are on the gravel when they sleep. I have kept an assortment of corys over the years and have never seen one attack anything.
I also know that there are diseases/parasites which fish can survive which then leaves them immune to it for some time. Ich is a perfect example and for so
me time research has been working on a vaccine.
I have a few fish in my tanks I got in 2002, I have other fish which are pretty rare and for which I paid a lot of money. I will never add something new to almost any of my tanks which doesn't get quarantined first. For tank raised fish this is a month and for wild fish it is 3 months. And this means that amount of time issues/illness free. I also understand a lot of hobbyists simply cannot do this.
I wish I could offer a better answer, but I cannot see nor observe the fish, so I cannot see something you may have missed. But the situation most certainly suggest and illness etc. and not murder. That conclusion is not real helpful as without having any idea of the cause I have even less of an idea for a solution.
The parameters seem OK, the maint. routine seems OK. The presence of plants is usually a plus and your filtration appears fine as well. I would rule out something toxic getting into the water as well since that normally kills most tank inhabitants pretty fast. I should mention that some diseases can be passed from a corpse to a healthy fish snacking on the dead body.
It can get uglier from here. You may lose the whole tank without ever knowing why. You might get desperate and take a best shot approach to treating. You might even get more than one shot at this. If you get lucky you might fix things. I have done this sort of best guess in the past a few times and had it mostly fail but it also succeeded once or twice.