I don’t like the term “self blue” because it could mean either lavender or blue without any lacing. Genetic accuracy could save people a lot of trouble, and I feel like they should change “self blue” to lavender in the Standard.
Yes, that’s because most lavender Orpingtons are hatchery quality. If they were “real” Orpingtons they would be bigger and fluffier, but since they’re usually hatchery quality, they are probably smaller than your Sussex.
Australorp legs are technically white, they just have epidermal melanin making them black. Same with Lavender Sussex. It is possible to have white legs on a black bird if it is birchen or partridge based, but this will make the plumage too light. But it’s impossible for lavender Sussex since...
It will be a genetic mess and mixed birds. Some lavender birds with silver leakage.
You could breed in black Australorps or black Sussex (if black Sussex exist.) It’s more ideal not crossbreeding, but it could be necessary.