In short both of these materials are a no go, there are few if any materials that replace rope normally. For society this is no go, however, for non-society play take a look at Sharkskin (a very unique material to say the least), whale sinew, black seaweed, possibly even whip wood. None of these items will affect its hardness greatly if at all, but they do allow for bonuses or alternatives to the rope/leather construction of netting. To my knowledge there is no limit to how many fortifying stones one can put on an object, and they do stack. But there are two drawbacks, the stone cannot be removed once used and it can be destroyed. A way that I've seen a GM limit the number of stones was to cap the number based on weight (i.e., 3 per pound of weight). The best thing I would say to do is get 4 fortifying stones at the same time, and then begin adding magical properties to your net as that will improve your item quickly and reduce the likelihood of someone destroying it before your able to reinforce it.
