Nico, The Summit Post picture you referenced is exactly what you needed to see on the way down. The high points are the Whitney "needles", and the low spots are the "Windows". If you click on the picture and enlarge it, you can see hikers in the lower left, and also near the first window. That is the trail you came up on, and the same trail you returned on. You were RIGHT ON TRACK! If you don't stop and turn around once in a while, the return trip often looks really different that the climb.
Your GPS track shows sort of a loop on the left side near the top end of your hike. The part to the left is the current trail route. The more direct short-cut on the right is an old trail, but you can't find it in summer unless you know exactly where to look. But right now, it is a snow field there, so you and your group likely followed all the tracks and climbed up to the summit using that route. On your return, you followed the obvious trail, and it took you on the round-about way.
The lakes you saw on the return were the same you could have seen on the way up -- Guitar Lake and Hitchcock Lakes.
The spike in your track is just an anomalous coordinate that gps units often record when you are in a canyon, where it can't pick up enough satellites to get a reliable location.
Again, you were on the trail, you just took a different variation on the way down. It is extremely curious how Marv got so far off course. It is possible that he took a right turn off that little spike on your loop at the top -- If there is snow there, he could have followed tracks from hikers climbing and descending the
"easy walk-off" route of the Mountaineers Route. Since there is likely a full snow field all the way down to Arctic lake, he probably could have descended on it safely to that valley. He would then need to circumnavigate around from Arctic Lk. (A man his age died there over a year ago thinking it was a shortcut down to where the Crabtree ranger station is. Marv had snow. The deceased tried it on bare granite.)
Here's a copy of your map:
