Q2) When the doubt between a co-ordinate and a double bound happens, there's always a chalcogen involved as a peripheral atom. In this case, there are 3, 2 O's and a S.
So that's the hint: First, complete the octet of the central atom with double bonds. Than, all the other bonds will be co-ordinate. In the ion thiosulfate, one sulfur is the central atom and the other is a peripheral atom. The central atom makes a double bond with an oxygen. With it, it's octet is complete, therefore, all the others bonds will be co-ordinate. However, because there is more than one bond with oxygen, there will be a resonance in the molecule. So, the following lewis structure is just a representation of one of the canonic forms of the ion: