Is fire-lapping worth the effort on a Marlin Guide Gun and a Marlin 1894P 44 magnum? If so, what are suggested loads to use  when fire-lapping?

You might want to get the Beartooth bullet guide. I just received mine and it is worth every bit of what the charge is and more. This is a rather large subject to cover here on the forum and is covered in the book in several chapters. I am really new to this kind of reloading and have learned so much that I will apply this knowledge for the rest of my life. The short answer to your question is yes. You can fire lap a Marlin. That is the rifle I will be doing shortly.