1.  Not my area of expertise but I think you have to have the lien on the crop for the year you rented the land to the tenant.

2.  I think one easy way is for you to notify the local elevators that you have a lien.

4.  Sounds like fraud.

6.  Sounds like you have already been defrauded.  I guess if you have a written agreement you can sue for breach of contract.