Hi,
Thanks so much for the kind words! Best way to contact me is either through here or twitter, but you can also try my email (person@alekssamoylov.com) - it's just so full of spam at this point that I don't really check it as often.
I will be the first to admit that I made that puzzle way too obscure. Though I suppose it is meant to be a sort of long term mystery, I think I sort of designed for myself more than for prospective players. Can't really promise I won't do something similar in future games (it might be like Miyazaki and poison swamps ... a part of me just can't resist), but I've definitely learned a lot from the feedback I got on this aspect of the game.
Anyways, I will tell you that you are very close when it comes to the Light and Shadow phrases. In both cases, you have the correct region, and in the latter case, you have the correct location (it is, indeed, somewhere in the cave). When it comes to the Light phrase, the precise location of the phrase is most directly pinpointed by the verse "you called for help, it came out wrong." I do recognize that if one doesn't live in the US, the phrase may or may not be interpreted the same, but I ended up including it as a Twin Peaks: The Return reference. The way you reveal the Shadow and Light phrases is a bit different than some of the others (they require an extra step), but once you figure out how to reveal one, it will point you towards the method of revealing the other, as the two are analogous.
As for the earth phrase, it describes a location in the "Alternate Desert" region (the empty, dark, nighttime version of the desert). It may require a bit of running around, but it should be in or near a spot that presents itself as a somewhat obvious "point of interest."