Did you modify it so the hotend it nearly the same length as the stock ones like I did?
As you can see in the picture (the one without the fan) I drilled the heatsink core so the copper heat-break can push up into the heatsink and the 2 screw flange now touches the heatsink.
I strapped two heaters in parallel just like you suggested.
I filled the thermistor hole with enough thermal grease that it pushed out when I inserted the thermistor. I don’t think there was room for any more.
My best guess, is that shortening the length of the nozzle has caused the heat transfer to be too quickly absorbed by the heat sink and and cooled by the fan, causing it be very difficult to reach 250 or so.
And having that heat travel up that copper is causing the filament to melt at the top of the copper instead of in the nozzle.
But I don’t know. Just a guess.
I suppose it could also be that the cheap thermistor is registering the wrong temp.