2. The Dialogue System evaluates all child conditions before playing the start node. It has to do this to know whether the next stage in the conversation is another subtitle or a response menu, since the continue button can be configured to do different things based on this.
If you're going to set a value in the start node's Script, add an empty node between the start node and the real child nodes:
LuaInterpreter (the default version of Lua that the Dialogue System uses) implements most of the standard Lua functions. You can read more about Lua 5.1 functions in the Lua documentation. This is the main documentation link, which has links to documentation in Spanish, English, and other languages.