I only bothered with digging when it was worth it or else when it was convenient, so I missed like 10-20% of the dig items, but only the unimportant stuff. And I clearly remember not bothering with that dig item in the west corner of Merseto.
I think the problem is total turn count (which is rarely a problem actually). The number is 100, and it refers to ally turns if I understand it correctly. If all your characters had the same SPD (and ignoring speed delay) then Rene must get it within 16 or 17 turns. She needs about 5 turns to get the other items, then she must cross the map from one corner to the other within 12 turns at most. Unfortunately she’s relatively slow, and when other characters overtake her, she’ll basically lose turns.
So you must make her faster and the other allies slower. In case you didn’t use her much, level her before the battle to gain some points in Spd. Give her a gale rune and remove this rune from others. Give her a godspeed anklet (if available) and remove these from others. Use slow characters like Eugene and Dario/Sigurd. I’m not sure about speed delay for laying beads and digging, but to be safe, just don’t do anything but wait when moving with Rene. The positive element requirement can easily be met with the remaining characters (only 10, should be easy).
"Within the four seas, all men are brothers" Shuihu Zhuan