When balancing a system, you open the furthest rad fully open and then restrict the flow as you you get nearer the boiler. Any that are hotter are restricted more.
Download the installation manual, will tell you all you need to know. Most installers don't bother rating boiler down, so you'll probably find its factory set to highest output, unless it was done right.
If its air, you will need to shut all radiators, then with the boiler/pump running open one rad and vent into bucket until it's hot, then close and repeat on other radiators.
If it's sludge, your need a powerflush of the system
And do the others get hot? Or try turning all the rads off apart from one of the cold ones, if that gets hot there's a circulation problem, sludge or air
Yes, unless you use a powerflush machine with internal heating element. As above tho, powerflush is only required on systems with problems or new installs