Both your doubts are valid
CSMA/CA will allow only one machine to talk at a time and reserve the bandwidth for that time. Ofcourse wireless is half duplex. But the positive side is now we have 802.11n standard with MIMO technology, 802.11n can get you more than 300 Mbps so even you are in half duplex and CSMA/CA is there effectivly you can get almost similar bandwidth as a wired machine. Before 802.11n bandwidth was a constrain for wireless but now it has improved a lot