は indicates the topic, that is the thing you have something to say about. Generally the topic has to be something that's known or familiar to the speaker and listener; otherwise what your saying would be... off-topic. It's like a vs. the. By marking 知らない人 with は, you would be saying "the unknown person" (or "strangers [in general]"), which isn't what you want to say here. You want to say "an unknown person" so you use が.

[I misread the next part and didn't see the かs, so i thought they were statements. Since I already typed this up, I'll stick with that. This would work more-or-less work the same with questions anyways]

In the second example you use は. The phrase "あの店" implies that the store is already being discussed. So it's perfectly reasonable to bring up the topic of "the store's bread" and comment that it is delicious. This topic is "familiar" to the listener because it's common sense that a store would supply bread. This is once again like English "the"; you can say the "the store's bread" without first explaining that the store has bread. In general, you tend to use は with adjectives. You can use が with adjectives, but it marks the subject as the focus... that is, it identifies the thing that answers some question. あの店のパンがおいしいです = It is the store's bread that is delicious. は vs. が here is like intonation in English: "The store's bread is delicious" vs. "The store's bread is delicious".