Aftermarket one should be fine.
Sounds like you have found your heater issue.
Hopefully the slight overheating is not a major issue.
Its been warm out. But like I say check the fan is working as it should be coming in and off as required. And fit a new stat as there cheap enough. And go from there.
Check the basics mate. Oil and Coolant levels (when cold obviously)
Make sure the fan comes on and goes off as expected.
And assuming there ok a new thermostat would be a good idea I'd say.
Hopefully nothing more serious.
As for your heater I'd suggest it's unrelated and sounds like a fuse may have gone. That would be the first place to look.