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Answer to Q1:   This is a complex question and there is not one simple answer.  However the short answer is we convert chemical energy through complex biological processes into heat and work.  Our digestive systems (which converts what we eat into energy and other compounds) have evolved over time.  In our digestive system we break-down complex molecules (food) into substances like sugar, which when combined with oxygen produces heat and work types of energy exchanges within our body.  We are able to processes some cooked foods better than most other organisms.

Answer to Q2 :  No.  Steady State Processes's do not have to be reversible for open systems.  If you think of your system being contained in a control volume which shows time invariant properties i.e. is at steady state, you should be aware that entropy can yet be continuously generated and dispersed through the boundaries of the control volume in a manner such that thermodynamic properties, like temperature, pressure, volume etc. are measured to remain constant within the control volume.   Reversibility means that no new entropy is generated.  A reversible processes for an open system is one that maximizes the amount of work produced because it does not create new entropy.   However, in reality, open systems (i.e. where mass is input and/or taken out of the control volume) are difficult to make fully reversible; although such an approximation is often used for solving problems.   Note that the entropy is not a conserved property unlike mass and energy which are conserved properties (especially at velocities that do not approach the speed of light). 

Answer to Q3:  For a reversible steady state thermal isentropic cyclic process, the change in enthalpy is equal to the maximum work that can be extracted (or conversely for a pump or refrigerator it is the minimum work that is required to run the device for the required objective).   Regardless, the thermal efficiency is still limited by the second law limitation.   For a cyclic thermal device which is adiabatic and has no changes to the control volume shape or size with time, one can obtain close to isentropic approximations; but not always a strict steady state condition when comparing the input and output thermodynamic conditions of the fluid entering and leaving the device.