Replacing the timing belt on an Interference engine, and doing it wrong, can cause serious problems. Busted piston, bent valve, cracked head, are a few.
It must. I am having my '94 Camaro's timing gasket changed for just that reason.
Bad thermostat, plugged or damaged radiator, loss of coolant, ignition timing is off, faulty water pump, head gasket... and who knows what other problems can cause overheating.
That isn't likely. Most head gasket failures are caused by overheating. Not changing the oil will cause the motor to sludge up and increase the wear on the internal parts.
Yes, it can cause hard starting.
Yes if the outside drive belts are over tightened after doing the timing belt it will pull up on the crank causing extra stress on the front bearing.
you have a blown head gasket water is getting in to the engine oil
Not changing the transmission fluid in a car within 200K would cause problems.
A common cause of problems in cylinder head gaskets is leaks in the gasket. When a leak forms, the gasket must be replaced.
try changing both timing gears
Incorrect timing can cause several problems. One of which is high idling. 5000 rpm sounds high though, there may be other problems as well.
The most common cause for no compression after a timing belt change is improper timing. Make sure that tdc is set correctly, and is not 180 degrees out of sync.
Possible cause is a leaking head gasket or timing chain gasket if the water pump mounts to the timing chain like on a ford 302 cid You probably are leaking into the crankcase through the headgasket. There is a slight possibility that the block is cracked.