How can I add multiple Parent categories to child categories? or how do I skip a level to display more categories eg I need all child categories to display on the home page but when the menu items are selected they show only the relevant category children
You can use a module to show your category and not use core Display. For this you can hide category in the view(tihs can be set in the virtuemart category item menu)
If you need You can use this module https://shop.st42.fr/en/categories-tools/virtuemart-categories-menu.htm that can render many ways the categories. big trees demo (http://pro.st42.fr/fr/taxonomy)