Bad repair from garage suggested by extended warranty repairer who refused to cover the fault

The JCW mini blew the second spark plug and needed to be repaired. The warranty company told me to have the car recovered to the garage they suggested and then decided it was not covered (spark plugs not covered).
I had no option but to get the car repaired there and told them to get on with it. They first suggested a heli coil done in situ as this was the cheapest option, however I decided that a proper repair should be done and the head sent away and all for threads were re-done. When I went to collect car the airbag light was on (not on before delievered)and was told to bring it back, but more so to check oil and water daily for the first 500 miles. I brought the car home Friday and left it on the drive. Today Sunday I went to check oil & water and found three quarters of the dip stick missing see photos. Also part of engine mount sheared off and odd size bolt used to secure the head, 4inch scratch on front skirt and repalcement spark pugs and ht leads all at a cost of £800. What damage could this missing dipstick cause and what would you suggest is the best way to resolve this problem. It has been suggested a complete engine rebuild but as I am not a motor techician I really do not know where to go from here.