Yes, to both.
To change all edges in one go, you need to select all edges. You can do that either using "Tools" -> "Select Elements" or by clicking one edge to select it and typing CTRL+A to subsequently select all edges. (Both apporaches work for other elements like nodes and labels as well).
To create a template style for nodes and edges, first create a user-defined palette section (see "Edit" -> "Manage Palette"). After that create a node/edge as usual, then change its style to your liking. When done, right click the node/edge to open its context menu and choose "Add to Palette". You can set the new template as default by double-clicking the corresponding entry in your user-defined palette section. Moreover, you can now select the new template for Excel import and properties mappings.