As a UK citizen you can choose to remain at London. But are you wanting to interview there for other reasons? it won’t be quicker unless things change between now and then, both London and Sydney currently have a ~3 month wait for an IL after being DQ’ed. https://travel.state.gov/content/travel/en/us-visas/visa-information-resources/iv-wait-times.html
Yes, your wife can go to the US at any time.
No need to change the address, you’ll just put the new address on the DS-260, you’ll also be asked to confirm it at interview and were you arrive in the US too. Bear in mind some people submit their I-130’s decades before they get a visa, so address changes between those two states are very common.
Good luck.