A: More or less, yes. WiFi works, in part, on the assumption that bad people aren't trying to attack it.
For example, a WiFi client that is having trouble communicating with an access point (AP) can send a broadcast requesting all other clients to stop communicating. This approach is usually occurs when congestion or interference is preventing normal data transfer; it is intended for occasional use. But an attacker that is anywhere close to the AP can send that broadcast in a continuous loop to all clients. Because non-malicious clients behave properly, none of them will communicate, assuming congestion. Boom: denial of service (DoS).
There are many other potential attacks, including frequency jamming and client spoofing. Some are illegal, but most are successful thanks to WiFi's open nature. So although you can hinder intrusions and prevent eavesdropping, you can do very little to stop a wireless DoS attack by a dedicated attacker.