If your obd2 reader has a data list compare the iat reading with the out side air temperature thy should be the same engine off(cold engine let it sit a bit). If thy are you have no more iat problems. The p0420 is a bad cat code but I'm sure you know that. It should not affect idle.

Do some research on idle adjustment. You basically unplug the iac and set the idle with the air bleed screw to a specific rpm. You then do the idle learn procedure.

I think the 98 accords have a recall for this. Ya there is a recall for the air bleed screw. You can call your dealer and you may get it fixed for free you will have to give them your vin and have them do a vin static.