Concerning 2, Textsecure is not available in the Openstore but you can install it from the Ubuntu Store.
There is no development going on currently on this app. A call for a new developer has been launched on this forum but unfortunately there has not been any response yet. I would love to see its continued development so I really hope someone will take it on.
I used the version available in the Ubuntu Store on my Nexus 4. It worked fine and I could message friends who have Signal installed on Android and iphones. However, it does not want to work on my Nexus 5. If you are using a Nexus 5, it would be interesting to know if it works for you.