why does it need "per sempre", wouldn't 'sempre' on its own be enough for forever (always) ?
Forever is not a "real" word, but simply for + ever. It's just lazyness that joined the two words.
So just like you can't say "Live with me ever", you can't say "Vivi con me sempre"
for = per
ever = sempre
Why is it using the "tu" conjugation when we've just learned to use the "lui/lei" form when giving a command in the 2nd person? Is it to do with being formal/informal?
First, because DL wants "forever", and second, because "goot" is a misspelling of "good".