With regards to Ochiltree: whether that second event took place is still up for debate; it was announced but as you say there are no results to be found anywhere, so possibly it did not actually happen. Weird place for a speedway of that size, anyway.
The first automobile race in Galveston was on 11/29/1906 (Thanksgiving), reportedly on a two-mile course at Denver Beach. The races gained more momentum when they were paired with the Cotton Carnival, each August from 1909 through 1914. It seems they used a variety of course lengths, including 2, 5 and 10 miles.
As for the location of Denver Beach, I've never seen a map with that name on it. The only describtion of some kind I was able to find is from a 1909 newspaper clipping which says: "the course (...) being from the west end of the city limits to about Thirty-ninth street."