quoted

So basically we are trying to use as many microframes as possible with as few packets
per microframe as possible.

Did I understand this correctly?

Yes, you are right.

quoted

How will devices react if they expect to get 16 packets every 16th microframe,
but they get one packet every microframe instead?

I think that the synchronous endpoint must specify its period by
bInterval, but can't specify how data should be transfered during the
period by the host, and it just only receives data passively. So the
device can receive data correctly in the case(bInterval is 5).

quote from usb3_r1.0 section4.4.8 Isochronous Transfers:
"The host can request data from the device or send data to the device at
any time during the service interval for a particular endpoint on that
device"

As I understand the 4.4.8 section it just means the device can't assume a fixed
time interval between transfers, meaning that the host can use the last microframe
in one esit and the first microframe in the next esit, but still only use 1 microframe
per esit.

Section 8.12.6.1 describes how a 11 packet isoc transfer is allowed to be split
to 1 burst of 11 packets, 2 burst (8 + 3),  3 burst (4+4+3) 6 bursts (2+2+2+2+2+1) or
11 bursts of 1. These are however all within the same microframe. Splitting the
transfer into several microframes in a esit kind of makes the whole interval concept pointless.

-Mathias