Hmm my knowledge is limited but I'll try to help
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Does the bottom hose get hot too? Any idea or receipts as to when the thermostat was last done?
I think the only way to bleed the system is take the cap off the header tank from cold, start it up and let it run all the air bubbles out.
As to the HG, have you checked usual things like blows hot air from vents? Checked if theres no gunk in oil and coolant is clean?
I'm sure someone will tell me what I've said is wrong lol.