Problem with coolant in oil, is that if you ran it with enough coolant in there for a long period of time, there is no fix for it except for an engine overhaul. The flex plate can be checked depending on which transmission your have, by either a access hole at bottom of bell housing or be removing the starter and peering in the hole through there. It is hard to see if the flywheel is cracked, but when it is making this noise, you can listen to it from beneath the vehicle. The noise may be amplified by the bell housing. If you have a access hole at bottom of bell housing, remove the plate and try to see of you can hear it through there. The convertor tend to cover section where the flex plate bolts to crank, that is where they crack and knock about.
Try buying a stethoscope for mechanics, a long thin shank screwdriver or vacuum hose can work also to listen to engine noise, by pressing extremity to a point on engine and other end in your ear. Walk around engine with the tool to your ear, try to hear a difference in loudness at different points to see if it may be the cam or crank or piston slap.
Piston slap would account for good oil pressure. If it was a crank bearing, I would suspect a low pressure reading when oil is warm and engine knocking like it is. A lifter discharging would account for noise also, hard to diagnose by internet. I saw that you think it may be misfiring when it makes the noise, try determining if there is an actual misfire, such as spark cutting out or a bad injector/fuel pressure.
No CEL light is not good at start up. You should the light coming on for three seconds or so then going out. This should be repaired if the light is burnt or the ECU check engine light is faulty.
I would suspect the flex plate to make noise always so this is just an idea, not really something I would investigate unless all else fails.