Mostly it sounds like the batteries were low. FYI I wouldn't recommend running the car while trying to jump start the bus or much else. For jump starting a small engine hook up the cables, turn off the charged car, then start the other one. For jump starting a large engine hook up the cables and high-idle the charged car to maybe 1000-1200 rpm for a while, maybe 5 minutes. Then turn off the car and try to start the bus. If unsuccessful, re-start the car and let it high idle longer to charge the batteries more.
The reasoning behind both these recommendation is this: a starter motor takes a lot of energy.
- It's a good way to overload an alternator. Overload can stress the rectifier diodes in the alternator, and overload stress causes early or possibly immediate failure. So that's why I recommend jump starting with the source engine off -- it prevents loading the alternator.
- The starter on a big engine needs a lot of juice. More than will be easily supplied through jumper cables and alligator clips, thus the suggestion to let it sit and charge a while so that the vehicle's own batteries can do most of the work. Also, alternators generally don't produce much power when the engine is at low idle; thus the suggestion to rev it up.
Yes, the coolant heater burns diesel to make heat. It has an electric fan to push the exhaust out and it also has an electric pump to circulate coolant. It produces much more heat per minute/hour/whatever than the block heater does. If the engine just doesn't want to start cold, you could run the coolant heater to warm it quickly and run a battery charger to avoid depleting the batteries. Or run the block heater alone for several hours.
A diesel vehicle is usually designed to start reliably in moderate cold without help. I'd guess that most should start in 0-20 F using only a healthy battery and the glow plugs or grid heaters or whatever start aid is built into the engine (which may be automatic or manually operated). Colder than that, or if the start aid isn't working, or the battery is weak, supplemental heat like block or coolant heater can be required. At those temps or warmer where supplemental heat isn't required, it will still make the engine start even easier. That can be convenient and it can result in faster cabin heat. If the engine would require extended cranking to start, then the supplemental heat can also reduce engine wear by reducing raw fuel washing the oil off the cylinder walls (and oil contamination with fuel) and by getting the oil pump running sooner.