Laying Indian sand stone patio slabs on uneven concrete base

2" is not out enough to worry about so just lay them on a wet bed of about 4:1 sand to cement. To cover a depth of 2" you need about 1 bulk bag plus cement to cover about 10 square meters. Once done treat the patio with Thompson's Water Seal as per instructions on the can and you shouldn't get any salt stains and they will be protected from other stains when food or drink is spilt and also from frost damage. Lay the bed of mortar a tad thicker than you need, lay the slab on it and bed to the correct position then do the next... Let it set for a couple of days then brush in dry mix of the same ratio. The mix and the slabs must be dry for this!!! Deal with the one or two slabs that have decided not to stick, there's always one or two no matter how many you lay! Remove excess mix and leave to set. If you have a pressure washer use that to add water by standing way back so that only a very fine mist lands on the joints. Then treat with the water seal.

Use Sharpe sand to level out the uneven concrete base

Level up the dips on the concrete with hardcore then lay the Indiana sandstone with mortar wet

I would get pea stone ballast and just lay slabs on that 6to 1 mix they will be fine.