It's an octave clef. It's telling you all the notes written are actually down an octave. Since the guitar is already a transposing instrument where everything is transposed down an octave, it's essentially showing you the actual notes being played instead of the implied octave transposition. So for simplicity's sake you can just ignore it and play as you would with with a normal treble clef.
The lowest pitch a guitar can play in standard is E2 which is below the staff on the bass clef. Typically in guitar sheet music they notate this note on E3 so they can use the treble clef and it's just implied that the note is down an octave. With the octave clef, the note is actually an E2 even though it is written where E3 typically is because everything is down an octave.
You second question has is actually addressed in this question. Pretty much there are two ideas happening at once, the eigth note patterns and the whole note pattern that can be perceived as two different parts. Pretty much you would just play the half notes for the duration of the half note and play the eight notes while the whole note is sustained.
If they were two different parts, one would looks like this:
And another would look like this.
Since you are doing both, they become what you see in your score.