Ever? Who knows. My crystal ball does not work that well.
And “gain” is a really weird term for rights acquisitions. Generally rights are not gained, they are usually purchased or granted (I guess granted is close to gained).
The last question… Doesn’t have much to do with Vertical, but hey YOLO ASK Vertical. Let’s see Legend of Kamui has been published in Germany (1995), Spain (1992), Brazil (1993), and France (2012). Red Eyes (Akame) was published in Italy (2009). Handbook of Ninja Arts (aka: Kagemaru) was also published in Italy (2012).
Why hasn’t more of his works been translated? I honestly do not know. There may be dozens of reasons why publishers have not tapped this well. However, I hate having to repeat myself, but while there are a few exceptions, classical manga works in English are generally vanity projects and are true true works of love that often lose money, and take a lot of time and effort to produce. Moreover, for older works like this, which were not published by larger publishers, good files might be hard to find, turning a simple translation and lettering job into a bit of an archaeological/anthropological project.
That said, given Sanpei’s place in history and the recent movements by people like manga scholar, gekiga and GARO expert, and respected acquaintance of mine Ryan Holmberg I wouldn’t be surprised if at some point in the future Shirato’s legend will be heard again in English.