The Moment DW is one of those few tents that you set up fly and inner together.
That is generally seen as an advantage when it rains because the inner does not get wet,
Apart from that I am not sure I understand the difference (re backpack getting wet during set up ) between it and other tents.
Reading it again I think that what you meant is that the pack does not fit inside the vestibule when the tent is set up (?)
If that is the case , you can slide the floor further towards the other door so that you have one larger vestibule .

If you intended to have the pack inside on the floor with you, then better to empty some of the content onto the floor next to the mat and store the pack at one end .
That will not work all that well if you are over 185cm or so or if you use a 63cm wide mat.