1. Strange's spell: This is my theory: remember how Peter kept interrupting Strange while he was casting the spell? His interference caused the spell to become corrupted and "open the doors" of the multiverse. Therefore, during the time that the spell was about to be cast, the doors remained open and small amounts of people poured into the MCU.
However, eventually the spell was cancelled. Because of that, the doors were closed. So the damage wasn't as big as it could've been, and the rest of the multiverse people were stopped from coming in. Basically Strange canned the spell which resulted in the "doors of the multiverse" closing. Preventing trillions from entering. (Temporarily.)
When you think about it, it's possible others were summoned. It's possible that, say, a version of Kraven was brought into the MCU but he did not spawn in NYC. So we simply couldn't see him.
2. Venom: Yes, the writers have confirmed Venom was ported over because of his multiversal knowledge.