>> Is it acceptable to make Bingo posts using material published before the event began? Because most of what I linked to there is older stuff, published within the last 3-4 years.<<

In general, yes. See "Blast from the Past" in Bingo and Achievements. However, each host sets the parameters for their fest, so details may vary. This only matters a lot if you're playing a fest that offers banners or other prizes for making bingo -- the community rule is that you can play any card at any time.

>>I did make a handful of cards with random prompts from your list, but I'd been thinking to use them on possible upcoming work, not considering what could be done with them based on previous work.<<

There's no reason you can't do both.