Going to guess Donaldson being off the market pushes both he and Ozuna into the $18 million per range (maybe even a little more given how many teams still seem to be "in" on them).
If taking on all of Seager's money limits the prospect cost as suggested and Riley alone could get him, it's at least a little interesting to wonder if the money's available to add both Seager and Marte. I wouldn't think so, but who knows after the way AA has spent so far this winter?
If the 4th (and possibly 5th as it turns out) year is what kept him from bringing Donaldson back - not the AAV - is there around $25 million available? Seager and Marte are owed $30.5 million each of the next two years, and if you add Marte you could let Duvall go and save the largest chunk of his $3.5 million this season with Culberson/Camargo handling the emergency/5th OF role until Pache comes up. That puts their "cap hit" number at around $27 million. If you add some filler with Riley and can get the Mariners to eat some money (even if it's just this season) you ought to be able to get that number below $25 million.
The big question (for me) would be exactly what will Marte cost prospect-wise? What would you have to add on top of Waters to pry him away?
I realize how polarizing his name is but if he'd take a pillow deal to try to rebuild value, a Seager/Puig tandem looks pretty good about right now - that would allow you to keep all the kids (other than Riley) and buy you the time Pache and Waters need. In 2021 you'd have all of the Hamels/Melancon/Puig/Greene/Markakis/Flowers/Duvall money coming off the books in the event you wanted to make a play for Bauer or Paxton.