Water – A line T-ing off the water heater drain line. What might it be for

What type of faucet do you have? If you have a pull-out faucet then you can detach the pull-out head from the hose (that reaches down under the sink) and see if the water pressure is good at that point. There are small parts in the pull-out faucet that also could have caught some debris (although cleaning them out may be a challenge). But at least you could have narrowed down where the problem is; i.e. is it in the pull-out faucet head.

I have a setup similar to yours and the radon installers put a foam sheet over a hole in my sump cover, almost like a thin sheet of packing foam. This is almost airtight but allows some suction for the fan. You could terminate the drain line over a small hole that was covered with foam or paper such that the the air would be sealed in, but in the case you get water from the heater it would drip through it. You could also put a check valve in the pvc line from the heater but it would have to be loose so a trickle of water would trigger it. A trap would dry out, although the HVAC trap probably dries out during the winter anyway....but the suction from the fan should override that small opening.