The other three beneficiaries don't want the land to be used without agreement, but if I have understood correctly the only people who could possibly have any interest in using it are the two adjacent landowners — nobody else has access to it. There's no evidence that either of them want to use it — certainly neither of them wants to buy it.
However, one of the adjacent owners is alse a co-owner of this land. We're told that the other three beneficiaries "would not sign it over to him free", which suggests that maybe he asked for that, or hoped for that. And perhaps the other three now feel that this indicates that he has some interest in the land, and as the only co-owner on the ground, might now feel that he can just use it, because who will stop him?
First point to note is that this is not the executor's problem, and he shouldn't let them make it his problem. This has nothing to do with the administration of the estate. If the land is not going to be sold out of the estate then it should be transferred over to the co-owners, and the transfer registered. Let them argue among themselves about who gets to use the land and on what terms; there is no reason to let this hold up the completion of the administration of the estate.
Who will prevent the co-owner who owns the adjacent land from using this land, if he is minded to? The other co-owners, is the answer, and if they're not going to do it then nobody will do it for them. As they don't get on, they probably won't be able to agree among themselves on measures to stop the land being used, so if the co-owner with the adjacent land wants to use it he probably, in practice, can. Which, in the great scheme of things, looks like a better outcome than the land being neglected, and becoming rank and weed-infested.
None of this is the executor's fault. The testator made the decision to carve out an an unmarketable property and leave it to a bunch of co-owners who don't get on with one another. That is now working out pretty much as you would expect. God knows what he was thinking.