Re: is it possible to memorize music without playing it through an instrument..
absolutely possible, I used to do this a lot. I wouldn't say it's better or worse than practicing with the instrument. But if you know the music well enough in your head to have it memorized without actually having played it on the instrument, you have accomplished something significant. When you play on the instrument you might be relying on many other subconscious things (muscle memory) to remember a piece. An easy way to test that is to play something on your main guitar, then play it on someone else's guitar, if you lose your place or forget part of the piece, then you might not have it learned as well as you could have.
Sight-singing while looking at music and mentally fingering the notes would be one way you could practice this way. It's better than nothing if you don't have your instrument with you and have to learn something, and it's a good supplementary way to memorize something. It kind of forces you to think about the music a different way, and might use a different part of the brain than when you are sitting with your instrument. Some people can find it a bit mentally fatiguing, but the more different ways you can practice something, the better you will have learned the piece, and theoretically, the better you will be capable of playing and/or understanding it.