Have you tried setting that brightness field for each pixel? I have not tried it myself, but since each pixel in the protocol specifies 5 bits of brightness, I suspect it may work. A side note is that the SK9822-based lights (an APA102 clone) do current-controlled brightness, while the APA102s do PWM-controlled brightness. You may see less flicker with the SK9822s when lowering the "global brightness".
As for using 16 bits for each colour, it's probably possible to figure out a way to convert a 16-bit value into an 8-bit value plus 5-bit brightness. I would probably start with thinking of those 5 bits as a multiplier. The caveat here is that the brightness field affects all three RGB values, not just a single one. That would limit being able to specify 16-bit RGB values independently.
And yes, the Teensy could control multiple strips. You could either use the SPI hardware or bit-bang the values. The various flavours of Teensys have one or more hardware SPI ports, so if you need to connect more strips than the Teensy supports for SPI, then you could bit-bang. In summary, the answer is "technically yes". Performance might be affected, depending on the number of strips and number of pixels you want to support.
