You misunderstood, "someone else came from outside" is for a potential future plan for F.U.G., not what happened before. If the past was so well for V, he would not really be up for revenge anyway

I believe if F.U.G. keep quiet about V's return, it may be for an ambush plan, but even so, that would have failed as Gustang and Urek (FHs and Wing-tree) now know it. Now, they should try to keep it quiet or hidden for security reasons.
As said, I can guess why Hansung and Luslec does not really tell others, and FH may just decide to keep it secret. For the rest, I don't know. And still no definite info for either information lookup or whistleblowing methods in the tower. The tower is vast, many of inhabitants are just existing somewhere without much care. There may be bottlenecks to hold or release information easily.
Yet, I am losing hope for them to be clarified in the future chapters considering how far we came and how much we know. Moreover, historical cleansing of V might not be the actual deal itself, if not be mentioned in the spinoff it will probably be 100 chapters earlier to know it. We'll wait and see. V is an interesting fellow, there will be several cool arcs regarding him and some more flashbacks. Simultaneously those will probably reveal much more about Zahard, too.