As I understand Wifi, it can never true multicast, as each device
holds a '1:1" relationship with the AP, and then receives the same
packet... then the next device connects and gets the same packet.
Your understanding is flawed. Multicast does exist, but like most management traffic on a wireless network it must run at the lowest supported base/basic/required data rate. By default this is typically the lowest data rate supported by the AP.
The reason why is that anything that is broadcast or multicast from the AP to surrounding clients must be sent at a speed that all clients must support and can receive reliably.
What you are referring to is a technique that many access points can employ, which is a multicast-to-unicast conversion. Because the radio medium is a shared medium, a multicast frame sent at the lowest data rate can take 300X or more "airtime" than a unicast frame sent at the highest data rate. This is often far more efficient than sending multicast as multicast traffic.
In your example situation on a default configuration AP (802.11n or newer), it could take less time to deliver 10 unicast frames to each of 10 devices (i.e. 100 frames total) at the highest possible speed than it would to deliver even one multicast frame.
Two other considerations for multicast on 802.11 that are often reasons to use multicast-to-unicast conversion are that multicast frames are never acknowledged whereas unicast frames are acknowledged and retransmitted by the AP if there is no ack (not to be confused with TCP acks, this is a L2 mechanism and part of the process for finding optimal data rates between AP and client).
Second, if any client device in the BSS is using power save mode, multicast frames are only sent periodically (based on configured DTIM and beacon intervals) to ensure that multicast frames are again received by all clients. With multicast-to-unicast conversion, frames go to clients immediately if they aren't using power save mode and only delayed to clients that are in power save mode.
there are multiple types of router/APs - "3x3" and "4x4", some with
MIMO, etc. what should I use as criterion for choice of AP?
First, you would want an access point that meets or exceeds the capabilities of your client device. By this, I mean that if your clients are 802.11n 2x2 capable devices, then you want at least an 802.11n 2x2 AP. 802.11ac will give you some advantages even with all 802.11n clients and more spatial streams never hurts as the extra streams are in other ways utilized.
Second, you would want an access point where you would have configuration options to disable/enable multicast-to-unicast conversion, select specific supported/required data rates for the ESS, and preferrably to be able to configure the DTIM and beacon intervals.
can I TRULY multicast over wifi to iOS devices? Is my theory correct?
Respectively, yes and no. You can do it, however unless you really know what you are doing, or just to keep things simple, you may get a more "real time" experience utilizing a mulicast-to-unicast feature. Failure to get the below correct for your situation can actually reduce the experience on the client devices.
For good performance you would need to adjust the required data rate upward as far as possible while maintaining a reliable signal based on your environment and distance of the client devices from the AP.
Since iOS devices do certainly use power save mode, you may then want to further tweak the DTIM and beacon intervals to reduce the delay introduced by power save mode. Keep in mind that these settings can also decrease the efficiency of your airtime utilization (i.e. lowering beacon intervals means more beacons, etc) creating more problems than it resolves.
is there a way to multicast instead of treating each packet/client as a unicast?
As I already covered, yes. As long as your AP either doesn't have a multicast-to-unicast feature in the first place or has an option to let you disable it.