Mosfet Thermal Resistance calculation
I have just focused on explaining certain things because, when you grasp them, your other questions will likely become irrelevant.

Junction to case thermal resistance tells you how warm the junction will get above the temperature of the case. If it has a value of 30 degC per watt and the device is dissipating 2 watts of power then the junction will rise 60 degC above the temperature of the case.

This figure is the main bottom line and, all we can hope to do is prevent the case from rising too high in temperature and causing the internal junction to exceed its stated limits. So, we use heatsinks attached to the case to stop the case getting very warm.

But there's another parallel path for heat to travel and that is directly to ambient - this is the Junction-Ambient Thermal Resistance and is usually a lot higher than the Junction to case thermal resistance. But, it's a parallel path and so every little helps.

Also be aware that the case and local ambient on the device will rise as heat is drawn from the device so you can't assume that the local ambient remains steady at the external temperature of any container for the device (I'm thinking a PCB enclosure here).