My van is getting into the red is it the thermastat

just make sure that the plugs and wires are for your vehicle then change each sparkplug wire according to its length, the sparkplugs if they are new are already gapped.

Air lock and radiator.

Yes. I'll try to explain this. The glow plugs work by electric resistance. When they are clean and new, they work very well as there is no crud or coating on each part. The material composing each glow plug deteriorates over time something like a sparkplug.

The amount of time it takes to heat up increases as the glow plugs fail. This puts a strain on the wiring feeding the glow plug circuit.

You can test each individual glow plug using auto jumper cables and securing one end to the base and the other is briefly shorted to the top of the glow plug using battery voltage

Be careful-they melt. As you will see, some stay dark(bad) and some glow brightly(good). Or you can just put in all new glow plugs and be done with it.

I wouldn't give up on it, but I would sure be P.O.ed at the mechanic- $700 for a bad diagnosis. Why would he think 3 injectors were leaking? Was it running rich? Did he do a scan for trouble codes? Did he check fuel pressure?
Probably something minor-anything now should be minor after $700. Well, at least you got new fuel injectors. Have some tests done, scan for codes, check fuel pressure. Carefully clean the MAF sensor with maf cleaner spray, or at least a sensor safe spray cleaner. It is delicate, so use care. Let it dry thoroughly before putting back on. About all I can think of, good luck.

are you installing the correct thermostat the right in the head or block
3 thermostats indicate incorrect fitment method
heater hoses run either way as the water is pushed through the heater core by the pump

check and see if the fan is working and check the fan relay

You may as well change the #5 Injector to start with. I imagine you rechecked the spark on that cylinder as well. Could be a cracked sparkplug too. Check routing of #5 Injector wire. If improperly placed it could pickup a pulse to inject from other cylinders and then fire constantly.
Most likely you broke loose carbon in the cylinder head when the plug was removed and then it blew carbon back to the Injector head, blocking it open.

ck all fuses and fusible links and relays

What kind of van is it? Sounds like the blower motor resistor.

Does it start right back up? Is it during a turn, or stop, or maybe only when ac is on or anything like that?