1. According to halacha, you are not allowed to say something negative about a store or its products. Bli neder, I will try to find you the precise source if I have a chance tonight.
2. I don’t know Yiddish.
3. I am not sure of the answer to your specific questions. I would assume it depends if there is something in your statement which can reflect negatively on one of the stores in question and can potentially cause them to lose business. I don’t see how the second example can possibly be loshon hora especially if you make it clear that you didn’t visit the other stores.
I would think that the first example might constitute loshon hora, so I would ask a sheilah.
Again, keep in mind that we are discussing writing something in a public forum which is very different from giving advice to a friend privately.
When in doubt, asking a sheilah is always best. I think that it is also a good idea to think about how you would feel if it were your store (granted that aitzah might not work for everyone, since not everyone is the same).
4. When I find the sources, they may discuss similar scenarios and answer these questions.
5. Ask a sheilah.