Hello Michael,
1. Your guess is right, you would have to make multiple scenes (using inherited scenes could work very well). However, you can also press Alt+Click to place and select the object, so you can quickly change inspector settings, and SPACE to immediately reselect the asset for another placement, which might also be a fast workflow if you only have to change a few properties.
2. Snapping works at absolute world position, so if your snap step is set to 1 and you are placing on the XZ plane, you can snap at the XZ coordinates (0,0), (0,1), (1,0), (1,1), (0,2), etc. The offset now helps you offset this absolute snap step by a certain amount, so if you want, say, an offset of 0.3 each on X and Z, your objects would be placed at (0.3, 0.3), (1.3, 0.3), (2.3, 0.3), (0.3, 1.3), etc. You can only configure an absolute step, so the offset cannot adapt to what exactly "half the object's width" would be with your current object, but if you only use objects of the same size, then the offset that you can configure should work fine.
I hope this answers your questions!