First, I can count the number of reservoir caps that needed to be replaced on one finger... it got lost by the owner.
Second, the level of your coolant should RISE when hot and FALL when cool; not the other way around. Something is very wrong there.
If you remove the reservoir lid and there is venting, you are pressurizing your coolant system from a blown head gasket. That is all...
If you are losing that much coolant, it should make itself apparent. If you are using the G-12 antifreeze, it will leave a tell-tale trail of pink foamy stuff or at very least a pink trail from the offending part, that is, unless it's going out the tail pipe.
If the coolant is disappearing 'without a trace', (which it never really does), then check tail pipe for steam, smell of antifreeze. If that's where it's coming from, then either you have a head gasket leak or you have a holed EGR cooler.
The EGR is easy to trouble shoot. Take it off and pull a vacuum on the water leads. If it's your head gasket, the most common leak is between the #2 and #3 pistons and the two center pistons will read low compression compared to the other two cylinders. The two center cylinders will compare in readings quite closely.
In summary, the head gasket will blow when there has been a severe enough loss of coolant to cause the overheat light to blink. Did that ever happen? They should call that light the 'you just blew your head gasket' light.
The warping that follows requires redecking the head gasket surface and usually, an align bore to straighten out the cam shaft. Unfortunately, when the head warps, it is not very likely that you'll get the head bounced back to perfect flat. Since the head warps completely through and the cam journals need to be within about .004", that requires a cam align bore.