No, they can add a spouse in an ongoing application AFAIK.
Yes, spouse can be added in application BUT -
1. If added after ITA and before AOR, points will be recalculated (if added as accompanying AFAIK) .Here is where OP is concerned about points decreasing from 484 to 481 --> better to avoid risk in my opinion even if 481 matches the draw cut off score. If added as non accompanying, spouse can be sponsored later which again is very time consuming.
2. If added after AOR, points wont be recalculated even if added as accompanying. This is the best case.
So better to delay marriage to post AOR and then add spouse post AOR pre CoPR.
In any case, before landing, change of circumstances like marriage has to be disclosed. It is an obligation.