In my country they can be used for commercial links with up to 4W (36 dBm) – but not SRD – and they can be used with CPEs in Test mode:

Sure, but other devices woud store more data which needs to be sent out, too, and if they allocate the channel, your CPEs need to wait a little longer, too, thus resulting in lower bandwidth for all...

Your expectation would be right if all devices (the foreign ones, too) would agree to use MAXtream. AirTime is not necessarily divided equally (unless all devices use a common TDMA technique), but...

Right, but it won't work with only one AP. Even if it would, the AP still would not get more AirTime than what is left by other devices active on this channel. The only change I see is to use faster...

The way it's usually done is this: But I don't know whether TL-R470/480T can be configured in this way – it would require to configure one side as a load-balanced WAN and the other as a load-balanced...

So you mean two clients CPEs connected to the load balancer providing a wireless uplink to a single CPE in AP mode which is connected to DSL? IMO this could work only with two DSL modems / DSL...