The layer mapping should work as indicated. In your example the load_gds maps layer 1 in original gds into layer 10 in Nazca. Make sure layer 1 exists in the gds to load though. If that was not the issue, it would helpful to explain what is different from what you expect the layermap to do.
As for making wider guides in a loaded gds, in most (all) cases it is better to not use scaling on optical components. Now, if you load a GDS your guides have no “optical” width or length attached to them. They are simply polygons without a meaning.
A way to approach this is as follows: In Nazca it is possible to iterate over all structures in the gds once loaded. You would need to write a routine to interpret each polygon (after filtering out the ones you need, e.g. by layer or cell), update it to your liking, and Nazca can place the updated polygon in the layout instead of the original one. The basics of that process is worth a tutorial, but I doubt if this “interpretation” is the path you want to go.
As for the waveguides, thinking about it, you probably just want to grow the waveguide. It can be done with the iteration alluded to earlier and grow the polygon with the extra width before placing it back. This (general) iteration process is to be added as a tutorial. Alternatively, select the guides in Klayout and use “Edit/Selection/Size Shapes”.