I made this kit about a year ago. The only difference I made to the instructions were botting it at 3 months and adding some extra oak during aging.
Now 12 months in it's tasting really good and is the most popular red I've made. It started being drinkable at 3 months and everyone who has tasted it really likes it. It's a really good everyday drinker.
Just one thing to note is that at about the 8 month mark it started dropping sediment in the bottle and when I opened one at the weekend there was quite a lot of sediment. I guess the grapeskins means this is more likely.
Yes I just add it as per the instructions right before I degas. I never add the Sorbate though unless I'm going to back sweeten at some point in the future.
Bear in mind that degassing always takes me a lot longer than the Winexpert instructions say...
The sorbate (when used in conjunction with potassium metabisulfite) basically kills the yeast and prevents any refermentation - useful if you plan on back sweetening your wine. No need to add it because you plan on bulk aging your wine.
However some people can detect the taste of sorbate in wine so I choose to leave it out as I generally don't back sweeten my wines.
If your sanitation is good and you are sure it's finished fermenting before you bottle then it can quite safely be left out. It does however provide some added protection against infection so can be useful in that respect.