1. The gemeinde tax is in addition to the cantonal tax.
1. / 2. It's not surprising the rate is higher, as your employers will base the tax-at-source rate on only one salary, the two together will end up in a higher band.
3. The Baselland site has a page with tax FAQ that Google translate makes a decent job of:
https://translate.google.com/transla...um-steuerbezug
4. ?
5. Unless BL is different to SG, you only do one tax return and it goes to the gemeinde.