help again

[W. Hsu]

on my overheating problem.

refresh: i just bought a 93 se-r and it has a jdm motor.  when i drive it
hard or hit the freeway, the temp meter raises which i suspect it is a
overheating problem.  note this only happen when i drive for 1 to 2 hours
+.

i went to my local nissan dealer today to swap the thermostat(they sucked
big time) and have my system flush.  1 hour later, they gave the car back
to me and TOLD me that it still overheats.  when i tried to drive home, i
notice the temp meter raises up big time (therefore, i didn't go home
right away).  i was pissed off because THEY JUST made the problem worse.
after talking to them, they told me that my car didn't even have a
thermostat to begin with and the previous owner was trying to prevent
overheating by taking OUT the thermostat.  so when they put in the
thermostat, i felt one of the hoses was totally cold and the other was
totally hot which told me that no coolant is even going thru.  so i told
them to take out the thermostat.  and they tried to give me this bs
that my car is going to blow up and they really need to diagnose
the car for a week....bla bla...

first of all, is it safe to run the jdm motor without the thermostat so
that coolant is constanly flowing thru?
second, i suspected that the previous owner didn't swap out the JDM water
temp sending unit sensor so it is actually reading Fahrenheit instead of
Celsius.
third, could "stop leaks" in the radiator possibly cause the overheating?
and could the "stop leaks" get into my engine, heater hoses...?  (the
previous owner kinda hinted me to get a new radiator but didn't tell me
the whole story?)
fourth, could i have a failed water pump?

thanks for reading this far.