So based on my admittedly rudimentary observations, it would appear that Spider is indeed a one-gene-two-allele-deal where the spider gene is dominant - so it would be interesting to learn if "spinning" spiders are more often homozygous than they are heterozygous, or vice versa (which would be a good argument for sequencing).
Further, the Pastel gene appears to be a case of "incomplete dominance" (as opposed to co-dominance), where the gene for Normal and the gene for Pastel compete to produce a "mixture" or your standard pastel. The pastel gene un-hindered by the normal gene makes your super-pastel.
In the case of the Mojave, Co-dominance would perhaps be more likely since the heterozygous form (the mojave) does not at first glance appear to be a "mixture" of a normal and a leucistic.
If anyone has an argument, please speak up.