Originally Posted by Karl Gustav Annus

No idea about the online booking bonus, but the MyWay benefit for Doubletrees actually gives you the missing 1k per stay So staying for free (albeit without breakfast).

Living nearby a 5k DT property, booking every other night is a good option for free Diamond for almost 2 years

Alternatively trying to maximise the length of Diamond using rollover nights - if one would book a room for 120 days of the campaign the result will be:
Cost 120k x 5/night = 600k points
Bonus 12 x 10k = 120k points
Bonus 1k points per stay
=Cost of 479k point, to get HH diamond for almost 3 years (Diamond would be reached in mid-February, then 80 nights would roll over to 2020, the last 40 nights would roll over to 2021)