You will want to add the fortifier when you have finished fermenting so as to get all the abv you can from it. You can also chaptalize(the act of adding small amounts of sugar during fermentation to achieve more abv). The reason this is done is because if you were to dump all this sugar in from the beginning you would shock the yeast to much and cause the yeast to not ferment. as for the sweetener you would do this after stabilizing your wine with m-meta and then sorbate. As far as adding more fruit I think it would be a good idea since you are trying to achieve a high abv on this, maybe go with an sg of 1.125 and add small amount of sugar at around 1.025(around 1lb of sugar) if it makes it down to 1.015 then maybe add another 1/2 lb of sugar. If it ends up a little sweet thats fine as this is a port and should be sweet. You probably will still want to do the fruit f-pac after anyways to add more flavor to hide the abv you are trying to achieve, Any idea what abv you are shooting for?