So I went through the exactly same thing and here’s my sound suggestion:
1. Book two, one-night reservations first using points: one 35K points reservation and one 40K reservation. That will be 70,000 points gone from your account.
2. Call Marriott CS to swap the 40K reservation using your 75-night 40K certificate: i.e., refund you 40,000 points, and attach the 40K certificate on the backend to the reservation.
3. Check online to see if: 1) your 40K points came back; 2) 75-night 40K certificate is gone; and 3) the reservation is good (has e-certificate attached).
4. Call or email the 40K reservation hotel to confirm that a proper e-certificate is attached properly.
This eventually worked... and then the corona virus happened and I had to cancel everything. (Was for Japan in April).