What I would do, if you really want to make the cultured version, is to make a hybrid. Start by inoculating with either straight Leuconostoc, or an LD or D culture, like Danisco's MD series. Then take your pH readings. When you hit 5.9, start heating up that cream. Heat it like normal, to normal mascarpone temps. As you heat, it will get thicker (use a good pot, and do not overstir... you want that curd to set as one mass). And then it should thicken enough to be able to form... well, it's kind of like a custard or a cream sauce. Looks the same as acidified mascarpone... maybe a little thicker. Then you follow through and drain per usual.
This would give you the best of both worlds. You would retain the low acidify, have better textural properties because of the bacteria, and you would gain an additional flavor from the bacteria, such as the added diacetyl. And of course, you would stabilize it by heating to such a high temperature, which would give you the desired shelf life.
For a faux, fully cultured mascarpone, what I do is up the fat from 20% to... say, 30-36%, and then culture with Leuconostoc all the way until terminal pH, which is usually about 4.7. I don't even strain it after that because it's so thick. It's definitely NOT mascarpone, but it's not exactly creme fraiche, either. Kind of a delicious, lighter hybrid. Sometimes what I do is use 25-28% cream, add culture, let pH drop to 6.3 or so, and then I will whip the cream to stiff peaks and let it continue to acidify. The bacteria stabilize the mixture enough to where I can flavor it when pH hits 4.8-4.9, and pipe it out as a type of topping, or use for my deconstructed cultured chevre cheesecake, my lemon curd mini croquembouche, etc.
Really, it's up to you, based on the textural and flavor properties you need.