Correction: it's a function of the signal frequency and bit depth (or noise level, whichever is limiting). Maybe I should update the article to make that clearer.
Galinstan is a less toxic alternative to mercury that is liquid at room temperature (and a bit lower). It's an alloy of gallium, indium, and some other stuff.
BubbleUPnP will talk to a stock FireTV using some other protocol, no sideloading necessary. There are presumable other apps that do the same, but that's the one I've used.
Linear phase is closer to an ideal reconstruction. For this reason alone, it should in my opinion be the default choice unless something else is specifically required.