when is it appropriate to use "is" for and instead of "agus" and why isn't it used here?
Just whenever you feel like it. Is (or 's) for and is colloquial. Weirdly though it's not technically informal as you will see it in formal writing from time to time.
Thanks so much!
Is 'sibh' used for plural you while 'thu' is for singular?
Sibh is for plural and formal use. So if you're addressing more than one person or a family member/person of authority you wish to be more respectful to.
Nice to see "Beannachd leibh" is accepted here.