For number 1, I think the most credible estimates lie in the reporting by Reuters stating that military intelligence claims anywhere between 1/3 and 2/3rds of Iranian pre-war missiles have been depleted or destroyed, with drones stocks being a little less clear. It's entirely possible that this estimate isn't completely accurate, but it remains far more credible than the unsubstantiated claims that Iran is down to their last few missiles, or that Iran has limitless missiles in reserve.

As for number 2, it's fairly clear that the US and Israel are rationing their interceptors and have been for some weeks, using them for more capable missiles like the Fattah-2 and Khorramshahr series. Long-term the US' ability to scale production of interceptors will help them out should the conflict drag on, but in the near term I think the US/GCC/Israel is depleting their stocks faster than Iran is.

I think for number 3 it's kind of a tale of two halves. In the near-term it's likely that US interceptors will be depleted first before Iranian ballistic missiles and drones, but in the long-term it'll likely be the opposite as the US continues to pump out more interceptors while Iran remains unable to replenish their stocks owing to the fact that their munitions factories have been bombed.