John's father's uncle is not John's grandfather! He would be John's great uncle, and a brother of John's grandfather.

I don't know that it was cold-hearted of him to leave the estate in this way either. Certainly it was not terribly considerate of Mrs. Dashwood and the girls, but it was also not uncommon in those times for landowners to try to keep their estates intact and passed down along the family name. By doing this, John's great uncle secured the estate as a whole for three more generations at least. What would happen after that would be up to young Harry Dashwood, but with parents like John and Fanny, the child would no doubt be inculcated with the notion of keeping all the land together and passing it down a male line.

The introduction to this Broadview edition points to the word 'settled' and the importance of maintaining that family continuity and keeping the estate together in the very first two sentences of the book:

"The family of Dashwood had been long settled in Sussex. Their estate was large, and their residence was at Norland Park, in the centre of their property, where for many generations, they had lived in so respectable a manner as to engage the general good opinion of their surrounding acquaintance."

Kathleen James-Cavan writes that this has the plotting benefit of putting our heroines in the maximum kind of predicament and peril, but that it was not at all uncommon in those times to tie up land in that way. And she points out that JA's attributing the decision to do so to Harry Dashwood's cute behaviour is ironic because it makes it sound like a whim of the old uncle's when, in fact, an estate that had been "long settled" for "many generations" was unlikely to be passed down in any other way. Maybe this was even more important to a man who was leaving this world with no children of his own.