1. There are procedures for dealing with executor disputes, which can include a Court removing someone as executor. You must have legal advice if you want to do this.
2. If the land was to be valued simply as agricultural land it should not have been a difficult job - or resulted in widely varying figures. However, it can be legitimate to include in a valuation what is called "hope value" - an uplift for the expectation of a possible grant of planning permission for development. It may be that that is what the third valuer did, but it should be set out in his valuation document.
3. I don't understand the basis of all this. If the land is owned by the Uncle how come the Grandmother's will has anything to do with it? If the land was owned by the Grandmother, how could the Uncle sell it when probate has not been settled?