Good Lordy he used no layer insulation on the primary! WTF, jus saying...
Hi Pot should be 1500 volts between primary and core and secondary and heater wind.
I like using two wires for the heater as it lays flatter and requires less tension. This helps keep the secondary HV wind from getting smashed by the big heater wire.
And it is good to start the secondary with the CT, that way you have ground potential next to the primary instead of the high voltage AC starts.
You can also finish with a CT which means the entire HV secondary has a pseudo screen around it which might cut down on e fields escaping.
But this is not a good idea if the HV secondary is over 300 volts, ie a 400 volt sec will mean 800 volt layers next to each other.
The heater wind, if it has a CT is also a shield if wound on the outside which it usually is.