I have read that you cannot absorb more than 100 or so mg at a sitting, and therefore the huge megadose pills are a waste. You are better off taking a small pill a few times a day.
Taking vit C for your immuse system is a waste of time. The largest study ever done was on vit C and the common cold and they found no relationship. Vit c does not increase immunity.
As far as the isomerism of synthetic vit C, i don't know. A stereo-isomer, is two molecules that are mirror images of each other. The example always used is your hands. Your hands are built the same way, same fingers etc, but are mirror images and no matter how you rotate them they will never have the same conformation. Many molecules/proteins have mirror images, but they cannot be used since every enzyme in your body (where the molecues fit, like a lock and key) are made for only one kind, the L isoform. Hence why everything is "l-glutamine" and "l-leucine" and whatnot. If it is the D form, don't buy it; but I have never seen a d form in a store before, so don't start checking all your supp bottles.
Since that info is common knoweldge to supp industries, I doubt they'd have a lot of the wrong isomer that cannot be used. I assume you are fine, but don't know for sure. It should be as easy to make one isomer as the other. I would think the worse case scenario you have a mixture of both and you only get half.
