Technically speaking yes you can use plain water in your cooling system but it isn’t recommended as a long term solution and certainly not in extreme weather conditions.
Checking your Brake Fluid is a step which can be easily missed from your routine fluid inspection but it is certainly worth checking up on regularly as with all the fluids in your vehicle because it is vital to ensuring that your brakes work and work well when you need them to.
Heating, ventilation and air-conditioning systems (HVAC) are susceptible to all sorts of problems which could be causing it to blow foul smelling air into your vehicle.
Before you set about checking the health of your antifreeze it is important to bare in mind that different vehicles will have different expected lifespans on any fluid that goes into the vehicle. Your user manual should give you a guide as to when you can expect to need to change your antifreeze but it is a good idea to check up on the health of the fluid when you check if it needs a top-up, something you should be inspecting every three months or so.