electricity questions?

another electrician here,,,,,,,,,,,,,,,,,if it were me,I would pull all of the existing wire out of the pipe and pull a heavier circuit, #6 gauge wire,good for 50 amps to the garage, set a small sub-panel with a 30 amp,220 volt for air compressor,,,,,an appropriate size circuit for the heater,,,,,two 20 amp,120 volt circuits for general use,,,which would include the welder outlet that you started this post about,,,,,,,,,,,but that's just me, good luck