Why would they start a legal battle?
They have no evidence, except for what Billy saw and what Elvis told them.
Apart from that, they didn't start a legal battle because that would of meant causing more undue stress and hardship to Vernon who was not well, and they would also be taking money from Lisa - which they did not want to do, as that was Elvis's daughter.
I suggest a bit of further reading into the subject, and discussion with people involved.
I think Vernon doctored the will.....which is why Patsy Gambill got Billy's copy from Billy, which had the correct details on it.
People tend to forget that Vernon spent time in jail for fraud and that he disliked Elvis's spending and generosity. With Elvis out of the picture, Vernon didn't want to give out money foolishy.
Elvis worked himself into the ground in the end, and did so in order to maintain Graceland and the guy's. The guy's all had families and Elvis felt a certain responsibility to them. It seems strange that he would continue to put his body under this stress while living, only to omit them from his will, after looking after them for so many years.
Everything isn't black and white, and research, not just on the internet, but actually meeting and talking with people involved yields dividends.
Some fan's can be naive in such matters or maybe ill-informed.