In the scenario you describe, you should definitely be looking at multiple access points, preferrably dual band APs.
While coverage may be sufficient, coverage alone is no longer the primary consideration when deploying a wirelss network. Client capacity, channel utilization, signal quality, and reliability are much more important and multiple access points will help with all of these.
By using 3 (or more) APs on multiple channels (1, 6, and 11), you will in effect triple the amount of airtime (bandwidth) available on your wireless network.
Additionally, proper placement of the APs will provide clients a closer AP with stronger signal, which will be more resistant to noise in the RF environment. This will allow better signal-to-noise (SNR) ratios which will translate to the use of higher data rates and this results in more data transmitted per "timeslot".
I would recommend placing them 2/3 to 3/4 of the way from the center to the perimeter, spaced roughly evenly. Try to get them in or as close to the highest user denisity locations as possible (i.e. conference rooms, etc).
Finally, the additional access points will provide increased reliability. With a single access point, if it were to fail or reboot for any reason, this would create a disruption in service. Having multiple access points should allow for coverage to overlap, allowing service to remain (if degraded) when you have an access point down.
In a wired CSMA/CD Ethernet environment, it is possible to detect a collision because there are separate TX and RX pairs (using the example of 10BaseT). If a half-duplex 10BaseT NIC sends a frame on the TX pair, but sees that frame is corrupted on the RX pair, the NIC detected a collision.
However, with an 802.11 wireless device, there are no "conductors," just antennas which do not simultaneously transmit and receive. When an 802.11 device is transmitting it cannot in practical terms listen for another signal transmitting at the same time on the same frequency. The reason for this is that RF signal strength drops off very quickly when transmitting.
Even if we built an imaginary WiFi device that could simultaneously receive and transmit, it will only be able to hear a downstream collision if the other device is using a much higher output power (either raw power or through passive/active gain of some sort). Normally its own TX signal will be too strong and "drown" out any other received signal.
So another process was required, resulting in the need for CSMA/CA.
Best Answer
The ARQN bit doesn't provide the sender with enough information to be used as flow control. ACK only means "I received the packet intact."
I don't see how it could also be used for flow control. If the receiver receives the packet, but wants the sender to stop, what could it do? If it sends an ACK, the sender will keep on sending. If it sends a NAK, or sends nothing, that tells the sender the packet was bad. So the sender will retransmit it, making things worse. The FLOW bit means "I'm busy, please stop sending."