I don't think that's right. Nothing outside Earth(2014)'s past light-cone can affect anything inside it, nor can anything outside Earth(2015)'s past light-cone

I don't think that's right. Nothing outside Earth(2014)'s past light-cone can affect anything inside it, nor can anything outside Earth(2015)'s past light-cone affect anything inside it. Expanding the light-cone into the intervening year doesn't require you to simulate anything further out - well, you need to go outside Earth(2014)'s light-cone to simulate Earth(2015), but that's hardly worth mentioning. I think you're picturing a sphere of 13Gly radius (or whatever) centred on present-day Earth, expanding at lightspeed to encompass new stars and galaxies. But while new matter enters our light-cone, it is not doing so as stars and galaxies. Those all have pasts within the light-cone - you don't need to go outside our past light-cone to find all the things that can affect them. Any matter that only entered our observable universe in the last year doesn't have a past, because it only just came through the Big Bang. It's primordial chaos, not fully-formed stars; you don't need to work out millenia of its past to know what's there. Unless for some reason the simulation needs to calculate pre-Big Bang conditions, which is possible, but then the definition of "light-cone" needs to be amended.