Well the fact that Reiju has been able to escape while being so severely injured, is striking. To me it looks like her assailant(s) intentionally let her go. One of the most likely candidates for this would be Capone. We know he is scheming against Big Mom, so intentionally attacking one of the Vinsmokes, but letting her go could potentially lead to a violent conflict between the two factions. After all Capone is officially still part of BM's crew, so if he attacks Reiju and she reports that to her father, well he could easily interpret that as BM betraying them and Judge really wouldn't let that slide. So during the ensuing chaos Capone could make his move.
That Pudding is behind this, is an interesting thought. Even though she has appeared so far as a harmless girl, she is still very well the daughter of a Yonkou. That she is stronger than she looks, is a plausible scenario. We also know she's planning to do something drastic and attacking Reiju could be qualified as such. She might be hoping that by doing the latter the wedding is off the table.
Putting those two aside I don't think there aren't any other concrete suspects. I doubt the Vinsmokes themselves would be behind this, the Strawhats wouldn't use such tactics and most of BM's subordinates are loyal and know very well how much this wedding means to her. So I doubt that Potato Starch would be the culprit.