Fitting kitchen whilst concrete floor still drying.

If the base units have the usual adjustable leg arrangement, any chipboard edges will be well away from the floor. I wouldn't worry about it

Bit of polythene sheet under the edges which are touching the floor, or PVC tape and leave the plinths off for a few more weeks (inch/week rule of thumb with concrete) to allow air to circulate under the units.

The basic rule of thumb for setting concrete is that after 24 hours you can walk on it, after 48 hours you can work on it, after 7 days you can build on it and after 28 days it will have reached in excess of 90% of its final strength (it will go on setting for years). If appropriate, structural engineers base their designs on 90% strength, so after 28 days, it is considered fully cured for structural purposes.

So, if it's been laid a fortnight, there's no problem building on it. As others have said, if you have plastic-footed units then don't worry at all, just leave out the skirtings (which you can't fit anyway until the floor is finished). Otherwise, use plastic (dpm would be a good option if you have any) to protect the cupboard edges in contact with the concrete.

Amazing how many people fit the plinths and then tile / laminate the floor.

Yep. Not how I would do it though. ;-)

Stick a piece of plastic over the slabs and leave for an hour to see what condensation is collected underneath. If loads, then leave for a while. If not much you should be ok, provided you do not put kickboards in place or have draught free areas under the cabinets.

All professional installers, IME...