I just had a 24'x30' slab poured for my
shop. Zero access for a concrete
truck. 6 guys with wheelbarrows can
move a lot of concrete in short order,
with no dry pours. They did a great job.
Much cheaper than a pumper.
Tell your contractor what you're up
against. He's ultimately responsible
for any damage to existing property.
If he wants the $ bad enough, he'll
figure out a way to get 'er done.
I also noticed from your last pic that
there is no footing trench around the
perimeter? Even with a steel building
that support is needed with the
expected loads on the finished floor.
Even if you have no frost worries,
you will have moisture in the ground
that will cause the concrete to float,
slip, and slide.