Melt and pour is not meant to have any oil added. If you add anything, should be no more than a tablespoon per pound!
Melt and pour is made with oils already
Sugar will always attract ants. and other critters I personally would not ever add sugar to soap, unless is on the saponification process which you can only do with cold or hot process method.
And yes, MP could last a year and ever longer if you only add color and fragrance. Fragrance may diminish over time. But if you add other things, shelf life is dramatically diminish. Additives fade, change color, etc. Specially something that if food that has an expiration date, should be considered when adding to soap!
Hope that helps.