Is it permissible to demand (one's share of) land that includes palm saplings and a building erected by the father, by way of Mugharasah, even though there was no declared agreement or specified duration of stay, and the father's actions implied his ownership of a share in the house?

There is an ambiguity or contradiction in the question; the questioner mentioned that the partnership is common between the two families, then stated that he had not heard explicit words about the partnership from his parents. In any case, the claim of partnership in the house and land is made by the questioner and his family, and the uncle is the defendant, because the house is in his name. The Prophet, peace and blessings be upon him, said: "The burden of proof is on the claimant, and the oath is on the defendant." So, either the questioner provides evidence proving his father's partnership, or he has nothing but his uncle's oath. The issue of mugharasa (tree planting partnership) indicates that his father did not own the land, as it occurs between the landowner and the one who works on it. Finally, the matter of inheritances and shared rights is serious and must be referred to the Sharia courts for investigation and verification.