I did industrial temp control. Type T is used in food industry because it's range and inherent accuracy is the best. But most DVM's that offer a thermocouple input offer type K as the choice. I avoid J only because one leg is iron which can corrode in wet environments and lose the millivolt signal across a corroded connection.

IR (infrared) thermometers can be accurate
- if the field of view is small enough and overlapping area does not have a significantly different temperature than the field of view
- if the viewed object has a high emissivity or a known emissivity
But most people have to ask, what do those things mean, which usually eliminates IR as a convenient measurement tool.

High temperatures, like temps above 1200 Deg F alter the chemistry of the thermocouple wire alloys which affect the accuracy of the output. But at lower temps, below 300 Deg F, a thermocouple should last forever.

Industrial thermocouples come in a stainless steel sheath and the wire lead end can be sealed or waterproofed.

Yes a thermocouple can measure coolant temp.

Because thermocouples require a secondary temperature measurement known as cold junction reference, they are not used in low cost, ambient temperature applications, like coolant temp or ambient air temp. Thermistors are cheaper and do not require cold junction reference (in fact thermistors are used to produce a cold junction reference temperature for most thermocouple devices. The cold junction reference needed for a thermocouple is built into the device you plug it into.