  We use a large data-set of realistic synthetic observations (PaperI) to
assess how observational techniques affect the measurement of physical
properties of star-forming regions. In this paper (PaperII), we explore the
reliability of the measured total gas mass, dust surface density and dust
temperature maps derived from modified blackbody fitting of synthetic Herschel
observations. We found from our pixel-by-pixel analysis of the measured dust
surface density and dust temperature a worrisome error spread especially close
to star-formation sites and low-density regions, where for those "contaminated"
pixels the surface densities can be under/overestimated by up to three orders
of magnitude. In light of this, we recommend to treat the pixel-based results
from this technique with caution in regions with active star formation. In
regions of high background typical in the inner Galactic plane, we are not able
to recover reliable surface density maps of individual synthetic regions, since
low-mass regions are lost in the FIR background. When measuring the total gas
mass of regions in moderate background, we find that modified blackbody fitting
works well (absolute error:+9%;-13%) up to 10kpc distance (errors increase with
distance). Commonly, the initial images are convolved to the largest common
beam-size, which smears contaminated pixels over large areas. The resulting
information loss makes this commonly-used technique less verifiable as now
chi^2-values cannot be used as a quality indicator of a fitted pixel. Our
control measurements of the total gas mass (without the step of convolution to
the largest common beam size) produce similar results (absolute error:+20%;-7%)
while having much lower median errors especially for the high-mass stellar
feedback phase. In upcoming papers (III&IV) we test the reliability of measured
star-formation rate with direct and indirect techniques.
