Ujm,
1. What is the difference? Stealing from anyone is prohibited by the Torah (SA 359:1 and see Be’er haGola 348:5). Please cite your sources. Actually,if the owner is non-frum/non-Jewish it might be worse as it might be a chillul Hashem.
2. Yes. It is a question. It would depend on whether people commonly change their minds and put items back on the shelf before paying. In most places that is the case so there was no kinyan because there was no completed intent to acquire. Probably the store owner does not care as someone else will buy it. However, where this is not common putting an item in one’s cart might be a kinyan.
Rational,
1. That was a method of the people of Sodom. Each one would steal less than a peruta’s worth.
2. True.
3. Finding someone a productive job is the highest form of tzedaka so there is a mitzva to help someone’s business.