Does Win 10 Miracast support “Multi-screen” sharing?

No.

I am using MAC book(IOS version 11.4) and connect to the same AP with IFP6550, but I can't find IFP6550 via Airplay, how can I do?

You need to power off then on the AP and reset it.

Does HDMI accommodate long cable lengths?

HDMI technology has been designed to use standard copper cable construction at long lengths.  In order to allow cable manufacturers to improve their products through the use of new technologies, HDMI specifies the required performance of a cable but does not specify a maximum cable length.  HDMI Org. have seen cables pass "Standard Cable" HDM...