RDR did lots of breedings of the normal looking platy offspring together and only reported normal looking offspring. Later he did platy X platy and got a normal looking female that should be a for sure homozygous hidden/dilute/whatever it's called. Don't know if he has bred it to a homozygous lesser to confirm that it produces 100% platy as expected.
So chemically I have no idea what could be going on to have so many different mutations of this gene. Hidden certainly is an odd allele (version of a gene) in that even the homozygous version looks normal but for breeding purposes just think of it as just like the other versions of this gene but just the most normal looking. If we ever do get DNA info on this gene I bet it will be very interesting (are there differing numbers of duplications or missing genes involved or some activity regulator or pseudogenes).
Heterozygous just means having an unmatched pair of whatever gene you are talking about. If the copy from mom doesn't match the copy from dad then the genotype is heterozygous. We often use it to refer to recessive mutant copies paired with normal but it's just as valid for co-dominant mutations (a pastel is heterozygous) and even two mutant allele combos like platy. If the two copies match it’s homozygous, otherwise it’s heterozygous (excluding the genes on the gender chromosomes that don’t come in pairs).