The food is indeed lembas but it’s not the only food the hobbits eat while on the journey. In the early going and at other times, Sam especially mentions how much he likes rabbit (coney) stew.
Yep, Tolkien mentions that in the same letter where he refers to the Host. It might just be my biases, but I don’t immediately recognise mother-worship as uniquely catholic in the way that bread with spiritual power is. I agree there’s plenty there if you’re looking for it.
It’s specifically in the last section of the journey, from the climb to Cirith Ungol, across Mordor to Mount Doom that they eat entirely lembas. The implication is that they wouldn’t have had the mental strength to make that journey if they hadn’t done so.
…and perform miracles of endurance in order to get to somewhere where you won’t have to eat soda bread. 
Yes, they don’t get their hands on lembas until they reach Lothlorien, and by the time of “Of Herbs and Stewed Rabbit” Sam is ready to admit that while it keeps you on your feet in a wonderful way, it doesn’t satisfy the innards, somehow, and what a hobbit needs is rabbit stew and taters - “the Gaffer’s delight, and rare good ballast for an empty belly”. And within a chapter they’re happily tucking into everything Faramir and his men have with them, and packing more to take on their way.