The last time you had to get 180/180 mission points to get all trader rewards three times. This time the points are very fair to optain. I already got 4x the points you need to get all trader rewards 4x and I'm still away from 100% completion. I hope future towers will have a random encounter boss too.
Yes, he may spawn if you re-battle monsters you already have defeated. He spawned several times while I tried to get all missions points on floor 1-20. He mostly offered me 3pts and rarely 2pts as rewards, never 1pt. He never spawned if I had replayed floor 10, 20 and 30. He may spawn anytime but mine spawned more often if my LP were below 4000 (maybe programmed that way intentionally to refill ..