Thanks for the kind words! Somehow simply stroking (in GIMP) seems to work only on a single path. After merging paths, I had to do path to selection, grow selection and then stroke. At least I didn't have to stroke every line individually.
I must have cocked something up somewhere anyway, I can select areas but they're just filled with transparency. If I want to work on a layer, I have to duplicate it, copy it over to another image, do what needs doing and copy it back.