Hey late to the party but you got great advice so far
I just did a coolant flush on the side of 401 yesterday

Didnt have any issues bleeding the system after a repair. I did have to run front heat to get coolant to the back heater but it all evened out. It went much easier than my 2F experiences.
Do remember that the factory needle guage is horribly innacurate and it often doesnt climb to the danger zone untill it is too late

You can search raventai temp guage mod if you like tinkering and making things better...............or pic up a scanguage/ultra guage and youll always be able to monitor temps... and read obdII codes and more.
Try roxx's test as well as your avalable kits at the parts store.
MTBcoach did a great video about a year ago on using the tester. I think he even showed how not to use it and the appropriate language to use in such a situation

Ill see if I cant find it
Sounds suspect but we will keep our fingers crossed its not. leak down, bubble test, HG kit and spark plugs and report back. You should be able to track down from there.