Summing up a bunch of comments, rather than littering the thread:
* There is no LN present in the final product. LN is literally nitrogen in liquid state, and nitrogen is gaseous at sane temperatures. You add it in relatively small amounts, and it evaporates away. It is absolutely not safe to consume LN.
* Yes: you need to ask your LN supplier if the LN is food-safe. This is an easier thing to verify than that a dewar is food-safe (because who knows where it's been). Our LN source also sourced other culinary applications.
* As I said in the post: guests didn't get to handle LN, which is good, because guests were intoxicated.
* The nitro-muddling technique in L.I. is designed in multiple stages to prevent that from ever happening; for instance, if you shake a drink with LN still in it, you'll hurt yourself before you manage to serve a guest LN. The final product has to be pourable. In fact, LN isn't a component of the drink itself, which is chilled with plain 'ol ice (for the dilutive effect).
* The ice cream recipe is freezing cream and egg and sugar, which are all eminently freezable substances (ie: they start freezing at a relatively high temperature). You drizzle LN into the bowl slowly while slowly running the mixer, and you stop when the mixer starts straining against the ice cream. You're never dumping so much LN into the container that it could settle without fogging the whole bowl up (if you do that, you end up with ice cubes instead of ice cream), so it's obvious when it's evaporated off. We remove the ice cream from the bowl with a silicone spatula, to give you an idea of how frozen (or in this case not frozen) the final product is.
(If it helps you understand, without having had the experience of actually doing it yourself: the first couple doses of LN into the mixing bowl are "eaten" by the custard base, which is still liquid. The bowl starts looking like a smoke machine. You wait for it to clear, so you can actually see what's going on in the bowl. You repeat until the mixer paddle starts leaving trails on the bottom of the bowl. Your intuition is that there could be actual liquid LN in the bowl, but that's not really how it works.)
* An example of an idea that freaked me out and I refused to let happen in my house: people freezing garnishes in LN and adding them to drinks. One of the clearest safety indicators you get with this stuff is "the product you're chilling isn't completely solid" (or, in the case of a drink, "at all solidified or slushy").