Low voltage grid frequency with renewable feed-in
If a low voltage grid's consumption equals to that grids renewable energy production, does it mean the transformator basically does nothing, like an open circuit? 


No.  They don’t act like an open circuit. They are connected to the MV and higher Voltages of the grid providing the Voltage reference (phase/frequency) to the LV grid. 
Grid-tied inverters need the grid as reference for voltage phase/frequency, if they dont sense any voltage from the grid size they wont generate. 


  Are grid-tied inverters (smart) enough to keep the frequency and the phase correct to that extent?


Previous answer already answers this question...
As addition to that , off-grid inverters need to be smart to sense the grid is off, and regulate the voltage/frequency level and once the grid is back, sync it back to the grid without interrupting the supply to the house. 


  How does one calculate a grids capacity to intake renewable energy, do you assume transformator works full capacity in the REVERSE direction? (pumping energy back to medium-voltage grid)


Actually for the transformer it doesn’t matter from/to which direction the current flows.. so one could assume the trafo can feed in from both sides almost nameplate power, BUT (and that is a real big but) there are other grid parameters that needs to be taken in consideration , as the Power Quality, to list some requirements please see list below:


Voltage level, according to norms a -/+ 10% Voltage deviation n tolerance from the nominal voltage is allowed, and multiple PVs inserting power to the grid will definitely increase the voltage level at the pvs connection point (house terminals). 
PV inverters need to comply to some norms and operators requirements and some of them include that if voltage grid is higher than 7% than nominal voltage, power injection must be reduced... so the limitation wont be the transformer but the grid operational conditions
If there is a frequency deviation the same applies.. but I don’t know the exact numbers..


Hope this clarified your doubts.