"- Why VMware shows RAW space (27.92TB) instead of just showing usable space (9.4TB)?"
Because multiple different Storage Policies can be applied to different Objects e.g. some data could be stored as FTT=0 and some data as FTT=1 (and obviously far more possible variations in larger and All-Flash clusters).
"- Can we block the use of space above the usable vSAN (9.4TB)?"
Unfortunately I am not presently up to speed on what ended up being implemented in 6.7 U2 with regard to 'guard-rails' but there are/were plans to start limiting things such as VM creation past certain thresholds. Either way though, from a administrator perspective there is no real control over "blocking" space.
"- Will any alarms be generated when near the usable space (9.4TB)?"
Yes, you will start getting warnings in vSAN Health once any individual disk passes 80% full and it starts rebalancing this data to other disks where possible:
"- If we go beyond the usable space? Will this cause what kind of problem for my environment?"
Like any storage system, the VMs need to be able to write to disk - if you run out of space on disks where these components reside then of course the VM will not be functional until space is freed up/rebalanced and it can write to disk. Obviously the solution to this is to size your environment accordingly and not run out of space.
"2 - Can you explain to me how vSAN does this 9.4TB calculation?"
It is very likely factoring in VMwares recommendation of allowing 25-30% slack space for temporary increases in used space (e.g. changing Storage Policy structure and of course snapshot-data during back-ups).