We have so many written sources, so much archaeological evidence of the Air Nomads, so so much agreement between them that honestly I'm not sure where you'd ever get the idea that the Air Nomads didn't exist prior to their revival. There's no evidence either that the Earth Kingdom either planned to invade the Fire Nation or had advanced weapons facilities, and they had no incentive to.

The Earth Kingdom was large, but very heterogenous and undeveloped, and in the years before the Hundred Year's War Earth Kingdom control over all its territory was not as strong as the propaganda would like us to believe. All of that meant that the Earth Kingdom wasn't concerned much with the world beyond its shores. If they wanted wealth or resources, there'd be somewhere within the Kingdom where they could find them. Rather than external colonization, they pursued policies of internal colonization.

Well this is certainly a novel approach by Air Genocide deniers.

Well, if the Earth Kingdom had actually possessed these research facilities prior to the war, then they must have still retained access to some forms of advanced technology after they were destroyed. If so, the Fire Nation wouldn't have been able to make so many territorial gains in the Hundred Year War as a result of their more technologically advanced armies. Actually, the Earth Kingdom military forces seem to have remained the same (at least concerning technology) throughout almost the entire war. Tank warfare wasn't even conducted by the Earth Kingdom until almost the end of the war during the invasion of the Fire Nation capital. There weren't even any factories in the entire kingdom outside of Ba Sing Se.

Also, why would the Earth Kingdom government place research facilities in the Air Temples when they could very easily hide these facilities in the crystal catacombs of Ba Sing Se or beneath Lake Laogai?