iiD tool has the software built in; you don't need to buy anything from the dealer.
Regarding your current air leak, it's harder to diagnose since you say it's been happening since you bought the car. Normally, if it's working fine and then one day the entire front starts sagging overnight, it's probably the valve block as it's unlikely that both front airbags would start leaking at the same time. If only one corner sags, it's that specific airbag.
Since you don't know whether the entire front started sagging at once, it's harder to tell. Regardless, Land Rover sells a rebuild kit for the valve block that's cheaper than buying the whole block.