I think you are all missing the point. Why speculate when Jesus told us. Two of the parables are explained. should not all the others be understood in the same light?
1-leaven IS a bad thing in the Bible. I think you are missunderstanding the verse. Three measures of meals refers to fellowship. In the mid-east culture this is well understood. Leaven would never be included. It would be insulting and shocking. Jesus was Jewish, His disciples were Jewish. We have to understand this from a Jewish perspective. The leaven is hidden and corrupts the whole lot. This is a WARNING from the Lord.
2-No all the kingdom parables are not 'bad' so to speak. The treasure is not bad. The treasure is the remnant of the body of Christ. The field however, represents the world which IS bad. The man of course is the Lord who did give up everything to purchase the field. And yes it is a corrupt field, but He paid the full price in order to get the treasure.
3-All these parables must be kept in context. Remember - Jesus explained two of the parables, the rest can be understood from this. Use the Bible for context, not our assumptions or imaginations. Jesus explained the two then asked if He had to explain the others. The disciples said basicly, 'no need, we understand now."(Matt 13:51)