No idea why rain should have any impact on that cave... never recognized such so far. Fruit will spawn randomly as long as there's enough free space, maybe you can increase the rate by changing the code, but that's way to technical for me to know about.
Should have nothing to do with it. Like terdin said, it has everything to do with empty floor space, and i believe it's a daily check.. I don't know anything about it but @BlaDe might know more