Maekar was said to be the Prince of Summerhall. That sounds somewhat like the Soviet top men's leisure places ("datsja's"), but I expect it is a castle somewhere. Where is this? Dorne?

Summerhall was a lightly fortified castle that Daeron II built on the Dornish marches, roughly where Dorne, the Reach, and the Stormlands come together. It was a Targaryen castle and a royal residence, especially when Daeron was young, but as he grew older he left King's Landing less frequently, and Summerhall passed to his youngest son, Maekar. (Baelor had Dragonstone, and Aerys and Rhaegel seldom left the court).

You will learn more about Summerhall if I write more Dunk & Egg stories.

[The rest: Was Dunk really knighted? What are prices in Westeros (800 silver stags to live a year comfortably, but food is super cheap)? How many silver stags to the gold dragon? How did Robert manage to leave the crown so far into debt?]

No comment on the rest, except to say that Robert was a generous soul who spent money at a prodigious rate. Look at the prizes he decreed for the Hand's tourney, for instance.