Sorry, can't help with the theory! Sounds far too complicated, no wonder you have gaps in your memory. I only remember learning Every Good Boy Deserves Fruit for the treble clef lines) and FACE for the spaces- forgotten the bass, don't use it in guitar.



Suggestion re your range: some songs need a wider range than others, and it can be tricky to work out the best key to sing in. Our Friday Folkies group has male and female voices, so we tend to do a lot of songs in D (a bit high for me, and the guys would prefer Eb, so it's a compromise; some of the women prefer to sing higher, too).



This is where a capo is so great - you can try out a key, then if it's a bit low, try it with your capo on 1st, 2nd, 3rd fret.. you may be able to go higher if your neck isn't too wide.



Try singing a song like Rock My Soul, which has a narrow range and only 2 chords - start in the key of D (D, A7 chords). For my voice it's OK in D if I sing low, but if I try an octave higher, it's too much of a stretch. If neither works for you, try a capo, or even a different key - the A and D7 chords are easy to play. I'll often try a song in the key of C (or D), then in G, to get a quick idea of a good key for a song - then I can use a capo to bring it up a bit if needed.



Once you get an idea of your favourite key to sing in, you can try that one first for a new song, then use a capo, or transpose if needed. I do find that my voice changes from day to day, too;) Some of our group songs have 2, 3 or even 4 sets of chords written in, as we've tried out different keys at different times. One group member used to call these one "alphabet soup".



Singing along to songs is a bit of a trap- so often the recording artists have a great range (they are professionals, after all) and if it's a male voice, that can be hard, too. I like Peter, Paul & Mary and Simon & Garfunkel to sing along to as I can usually do one of the voices. John Denver is OK for me, too. Otherwise I have to turn up the volume to drown me out! Perhaps you can find a good "sing-along" artist for you - or just accompany yourself on guitar :)