Advice on appropriate rail passes and/or tickets

Welcome Peter! The Swiss Card would be a good option. It provides a free trip from Geneva Airport to Lauterbrunnen, and from Lauterbrunnen to Le Châtelard (close to Martigny). All other trips during your stay will be discounted by 50%. Please find details at the Swiss Card page.

Another option would be to purchase a Swiss Transfer Ticket for the trips Geneva Airport-Lauterbrunnen and Lauterbrunnen-Le Châtelard. You could combine this with an 8 day Regional-Pass Bernese Oberland. If you plan to use public transportation in the Bernse Oberland quite intensively, this might be a better option as the Regional-Pass provides free traveling on a vast network, even including some mountain trains and cable cars (further mountain transportation is discounted). As the pass is valid on 8 consecutive days, either the 1st or last day of your stay in Lauterbrunnen won't be covered. This isn't a problem if you plan to do no or limited traveling then.

Only detailed calculations based on an exact list of plans would prove which of these options is cheaper.