  Spider is a balloon-borne instrument designed to map the polarization of the
cosmic microwave background (CMB) with degree-scale resolution over a large
fraction of the sky. Spider's main goal is to measure the amplitude of
primordial gravitational waves through their imprint on the polarization of the
CMB if the tensor-to-scalar ratio, r, is greater than 0.03. To achieve this
goal, instrumental systematic errors must be controlled with unprecedented
accuracy. Here, we build on previous work to use simulations of Spider
observations to examine the impact of several systematic effects that have been
characterized through testing and modeling of various instrument components. In
particular, we investigate the impact of the non-ideal spectral response of the
half-wave plates, coupling between focal plane components and the Earth's
magnetic field, and beam mismatches and asymmetries. We also present a model of
diffuse polarized foreground emission based on a three-dimensional model of the
Galactic magnetic field and dust, and study the interaction of this foreground
emission with our observation strategy and instrumental effects. We find that
the expected level of foreground and systematic contamination is sufficiently
low for Spider to achieve its science goals.
