There seems to be a sort of alignment of the stars at the moment.
First, Hilton has already extended status to March 2021/2022, depending on when your status was already due to expire. Then there's rollover nights, where nights earned in 2020 will rollover into 2021, effectively giving a member two years to qualify/re-qualify. And Hilton's current promotion means double elite nights.
American Express also has a spend $250, get $75 back at select Hiltons in Australia until 30 November (
Open for Travel | American Express Australia).
Bring all of that together with the status challenge and I don't see why it wouldn't be possible to earn Diamond through March
2023 with only nine (actually, probably 11 actual) nights and a few hundred dollars, if starting the challenge anytime from October on a date that would take your 90th day into 2021. Automatically, you only need nine nights instead of 18 to keep Diamond.
For example, start on 5 October, spend
eight nights at the DoubleTree in Northbridge (WA) for about $105 a night (if booking eight consecutive nights) or about $116 (if booked separately) for a total of $840/$928. That gives you 16 nights due to the double nights promo, just two nights shy. Use the Amex offer to remove at least $75 off that amount but tap your friends to use their Amex to knock $225 off (3x$75), bringing the total to $615/$703. You're left with two nights to reach the 18 required for the challenge. You could do just one more night before 31 December (due to the double nights promo) but that would only give you status to March 2022. If you waited until January and then did two more nights, you'd have 27 months of Diamond for, I reckon, somewhere around $1000.