only reason to get rid of pick 12 for yarran is by getting pick 21 from them as well and then on trading 19 and 21 to gws for pick 10 as the points system may help
If GWS are open to it (they expect their academy kids to receive bids from clubs with early picks) then that would work.
Pick 10 1,395 pts
Picks 19 + 21 948 + 878 = 1,826 pts
Alternatively, Carlton are after Tomlinson so you could do a 3-way trade:
Richmond - GAIN Yarran & pick 10; LOSE picks 12, 19
Carlton - GAIN Tomlinson; LOSE Yarran & pick 21
GWS - GAIN picks 12, 19 & 21; LOSE Tomlinson & pick 10
It would also be tempting to go after Carlton's pick 8 with a slight modification to the above in terms of first round picks.
Richmond - GAIN Yarran & pick 8; LOSE picks 12, 19
Carlton - GAIN Tomlinson & pick 12; LOSE Yarran, picks 8 & 21
GWS - GAIN picks 19 & 21; LOSE Tomlinson (GWS keep pick 10)