Help? P0300 after p0306, I changed sparkplugs and weird overheat issue
 

So I bought my g in may of last year I've had about a year and a half, no issues other than my cam sensors needing to be replaced.
about 2 weeks ago I noticed my heater stopped working, the next day I took a trip to get lunch and back and then got home parked it in the driveway after idleing for about 5 min I notice my temp gauge shoot up and then coolant steam came from my engine bay I pooped the hood and it looked like I sprung a leak, had a buddy of mine look at it, a certified Ford mechanical and we could not find a leak and the pressure tester I rented was faulty but now fast forward to 2 days ago, I noticed when I ran my diagnostic I had a p0306 figured I'd change the spark plugs since I just hit 120000 last month while I had it apart putting in a new thermostat.( I honestly think the overheating issue is because air got into the system because my overflow tank cap was not put back on after I checked my coolant probably a few days before the overheating issue started and my heater stopped.
Well I haven't been able to try to burb the system to check if it's just air yet due to getting p0300 code now after I changed the spark plugs, when I did I noticed very little gas maybe oil at the bottom of 6 and 2 but I figured that could be from the misfire that was occurring and I mean very little not puddling or anything . I also move the ignition coils around but I don't know if that would cause multiple misfires I believe the connector are all correct on the coils but I think one is bad because I'm getting no resistance when I take the volt meter to it's connector ( the connector on the coil not from the wiring harness)
The only two codes my car is throwing are p0300
And p0430 I'm wondering if build up in the cat is could he causing my overheat as well. Any help would be great appreciated hopping it's not a head gasket but it's show now signs of it, no coolant in oil or vise versa, no white smoke, no water coming from exhaust etc.
I'm playing on buying all new ignition coils today to try to fix the p0300 hoping it was just the one I think it is ( I'd rather replace all if I'm gonna do that one) my car has had no previous issue until 2 weeks that I've noticed and I drive probably 4-5 days. a week no more than probably 30 miles a day on occasion maybe more.

All spark plugs are iridium Safelite brand all gapped to .044

Oil in spark plug hole can cause misfire(but obviously a bad coil will too); you should of addressed that issue.

Easy fix- Clean out plug holes and spread(with finger and glove since it's very sticky) RTV-1 minute black or gray gasket where valve cover meets head.

Harder fix. Valve cover gaskets

As for coolant. I personally would drill a small hole in the top of the thermostat housing(some actually come with this already I believe), but I've never had a problem with air in my coolant system

if there is no warm air coming out of your heater, then there is no hot coolant circulating through it. I would try to address that first. That's asking for trouble. Make sure you have topped off the fluid in your radiator system, try to burp it the best you can (fill it then squeeze and release the large coolant rubber hose on the thermostat side, which will force the liquid to move around back and forth...you should see some bubbles come up in the radiator as you do this....obviously do this with the car off and the radiator cap off). There is also a bleed screw on one of the radiator rubber hoses at the back of the car. If none of the bleeding helps I would just replace the thermostat next.