Engine problems with my 2002 KA collection?

purchased a 2002 KA from Ford dealer last year, one or two minor probs which were solved within the 3 month warranty (46000 miles). One year later (now 49000 miles) MOT and full service it failed on shock absorbers. Garage replaced those but driving home there was no power, and the car was very noisy-took it back to the garage said that they would put in a new engine (as a gesture of goodwill!) They told me that KA had been sold to me with the wrong spec oil and when they put correct spec in it caused the engine to fail?! could this happen? New engine put in - 3 months on have had to put in 3 litres of oil - garage had car back again and now replaced pistons and rings - had car back again and now it conks out when you pull over which they have told me is the (I think) IAC valve which is all coked up but is not under warranty because it is not replaced when the engine was. Any advice, here am without a car more often than I am with one - have they wrecked my car or is it right that the new engines are really bad?