here's the scoop on getting united miles for hotel - not sure about getting them retro before even joining Marriott Rewards. But maybe if you did join Marriot rewards, you could call their corporate number and try to sweet talk them. I got miles retro for about 2 months back but was already a MR member. See link below for info on the miles for stays from mileage plus.
stays.
https://www.united.com/web/en-US/con...telandcar.aspx.