Good article. Doyle would have to know of Preston to base Watson on him. The article only supposes that he did know of him. In "Dangerous Work", Doyle, on his return to Scotland on the whaling ship Hope in 1880, writes in his diary on reading the news in the paper on the battle on Maiwand. He may not have meet Preston, but could have read about him.
That could have been very possible. We can tell by his writing that he was very well read on current events.