firstly your mechanic is a weed smoker - his excellent prognosis of because Tstat valve is removed, water circulates all the time and evaporates, THIS ALONE is excellent material to win the darwin award of "interbreeding decays IQ levels" - The system is closed evaporation can only occur when there is air. Do you or he think that water can evaporate into vacuum??
anyway - point 2, the sound you hear in the morning when you open the cap is not pressure its vacuum in the system being filled with air. There will always be a little vacuum in the system as the vacuum release valve has a spring on it and requires some force to work. The 50ml you see as empty is probably some old hosepipe expanding from the vacuum being removed.
The reservior you see is an expansion tank, it collects the "expanded" coolant from the system via the radiator cap - the 1.1 bar you see is the pressure rating of the system, anything above 1.1 bar will be expelled to the expansion tank. When the system cools down it will suck it back through the little "vacuum hat" in the middle of the cap (the little valve all radiator repair guys in Pakistan assume is the only valve that works. WRONG - the thick spring that compresses when you close the cap is what sets the 1.1 bar pressure point, the vacuum hat is pulled inwards when the coolant contracts back and hence coolant is sucked back into the system.
If there is any sort of tiny leak in the expansion tank pipe till the bottom of the expansion tank then instead of the system sucking water back it will suck air - just like you cannot drink coke from a straw if there was a hole in the side of the straw exposed outside of your mouth and above the liquid.