Can any vehicle expert answer these questions?

I don’t think anyone can answer your questions, John, because one would need to see the car to do so.

My advice is to visit the dealership and put your points to the owner/general manager and demand to see evidence. At best this sounds to be general incompetence.

I suggest you use a different garage next time.

As above. Go back to the dealer and ask to see the evidence. Assuming none is forthcoming ask to see the Dealer Principal and put your concerns in writing copying trading standards and the companies head office. And name and shame. 

I stand to be corrected but I have never known an oil dipstick that was made of anything other than metal? Almost impossible to break a piece off? Have you checked the dipstick yourself?  With the tyres you need to challenge them with the information and get them to explain. A spare wheel, depending on age, can deteriorate, especially if in an underslung carrier but to say and unused tyre has lost tread seems odd. 

I have owned 4 Kia cars, from new,  for the last 17 years and used the same main dealer with little cause for complaint. They have recently changed hands so I will be watching carefully to see if things stay the same. Fortunately the long warranty period does help stop nasty surprises! 

David

Some manufacturers use a plastic piece on the end of the dipstick , Kia is one of them on some model years 

If they are a main dealer surely they have a spare dipstick to measure the oil? Can't see how a non used spare can have lost tread and by so much as well.

I too would demand to see the evidence.

Just got the car back from the dealers complete with paperwork.  They are showing the tread depths to be:-

N/S 3   3   1        O/S 1.6   3   3  with a quote for a new tyre.

I've just checked the depths myself using my Vernier depth gauge. My readings are:-

N/S  4.0   3.5   4.0   4.0    O/S  4.5   4.0   3.75   4.5

All measurements are from outside of tyre to inside.

Had I been my neighbour who uses the same dealership, she would have taken their word as gospel and bought a new tyre from them totally unnecessarily.

I think I've taken my car their for the last time.

And the spare?

Did you not make a fuss, John? I’d have done as I described before and wouldn’t have paid the bill in full until satisfied.

The spare in my XTrail although being full size is steel rather than alloy and has temporary use tyre on it. I assume just to save costs. The temporary use  tyre only has about 3 mm of tread. Perhaps the OP’s is the same and whoever did the measurements assumed it was a standard tyre.

More To save weight. 

I would have stopped using that dealer 5 years ago.