If there is a genus under anther code with the same name, then we do indeed use different disambiguators, but where there is only one, " (genus)" is by far the best disambiguator. The reader should not have to know beforehand what type of organism it is, and should not have to guess what level of the taxonomic hierarchy has been used; it could have been "Holmia (animal)", "Holmia (arthropod)", "Holmia (trilobite)", "Holmia (olenellid)", "Holmia (Holmiidae)" or any number of others. "Holmia (genus)" is the most predictable, most straightforward and broadest disambiguator; I have thought about it a lot, and I am firmly convinced that this is the best way to disambiguate articles on biological taxa.
Some of your other questions should be answered at WP:DAB (especially WP:DABNAME). Where there is no clear primary topic, the disambiguation page is placed at the unqualified title, although there should always be a redirect from " (disambiguation)"; there should never be a redirect from "" to " (disambiguation)".
The hatnote at Holmia (genus) is faulty, and should be removed. This omission was an oversight.