Is the toilet operating properly

The overflow tube drains directly into the bowl, not the tank. The only way I can think that it would prevent the tank from filling is if it was cracked or had a hole in it and allowed tank water to drain into the bowl. If this was the case, you would notice the fill valve cycling if it was a slow leak or never shutting off if it were a large leak. The reason the tank fills quickly when you divert the bowl refill tube into the tank is that you are simply not splitting the water from the fill value between the tank and bowl any more.

It sounds like your fill valve assembly is the problem. The fill value has 2 outlets, one that directs into the tank, and the other that sends water into your bowl refill tube. My best guess is that something inside of the fill value got clogged with sediment, hard water deposits, or something else that restricted it. These are relatively cheap to replace and likely not worth the time to clean them out in my opinion.

1. Are you sure that 100% of the fill water goes to the overfill tube? Look closely, most fill valves fill the tank through ports at the base of the fill valve which are not readily apparent to casual observation.

2. If your toilet keeps running and the "float valve doesn't reach shut-off level" don't blame the float valve (aka ballcock valve), it's doing what it is supposed to do. You should be looking at the flush valve.

3. Anyhoo, you need to find and fix the root cause and I suspect something other than your septic lines. A properly functioning toilet cannot overflow because the fill valve directs too much water into the bowl, it is simple physics. I suspect something else, such as a partially plugged toilet trap. It could be the septic lines, but the whole house would be backed up and you did not say that was the case.