Usually indicates a problem in the emission control system - have vehicle scanned to determine the problem
No there is not a water jacket in an intake manifold they are around the cylinder walls in the engine block.
The water temperature sending unit is on the front of the engine. It is next to the top water hose inlet connection.
An engine scanner will give you exact water temperature and gauging by it you can tell.
The water pump is run off the timing belt. did you have the belt installed by a Toyota certified technician? if not i would definitely check the water pump seals.
have no idea
It should last at least 100,000 miles or more but there is no guarantee.
I used to do alot of wheeling and any part axels, engine, tranny that was submerged in water the fluids must be changed.
It's best not to wash an engine unless you have to. If you have to it's best to use plastic bags to cover electrical components such as the alternator and try not to spray high pressure water into electrical components eg: intake, fuse boxes etc..
This engine has a timing belt. Needs to be changed at 90,000 miles. All seals and water pump should be replaced at the same time.
Try replacing the thermostat.
If it is a 4 cylinder the chain requires no maintenance or replacement. The chain will normally last the life of the engine. If it is a V6 change the cam belt and water pump at 90,000 miles.
The head gasket may be leaking. Try having it changed.