It could have to do with not having a thermostat installed, yes. I'm not sure if the IACV and coolant temp codes are related, but they could be.

I'm not entirely sure what the strategy is that the Honda ECUs use for determining the transition from cold to warm, but that may explain the ECT code. The reason I say that is if you don't have a thermostat and the engine never really warms up because of it, the ECU may decide it thinks the ECT sensor is bad.

Also, if the FITV is never properly closing because the engine isn't coming up to temp, the ECU may think that the IACV is bad because the FITV is still letting too much air bypass the IACV. Basically the IACV can't regulate idle properly because it's out of it's range of airflow control. SO again, you might be fooling the ECU.

You really shouldn't be running the engine without a thermostat though, it's really bad for engine efficiency if (depending on the ambient temp) the engine isn't warming up.

My diagnosis: Forget about the codes. Put a thermostat in, reset the ECU, and then see what happens. Then you can actually troubleshoot if the problem is still there.