Yes, the front picatinny section of rail is an M-LOK picatinny rail that can be removed if desired.
Yes, the handguard is octagonal with M-lok slots on each side including the bottom.
If by front sight assembly you mean the M4 style front sight/gas block, no this will not fit. If you have a separate gas block that isn't low profile and a front sight that mounts on a 1913 rail, then it should fit just fine.
The weights are as follows:
10" handguard: 7.5 oz
12" handguard: 8.5 oz
15" handguard: 9.5 oz
It does not come with the barrel nut, instead it uses a standard milspec barrel nut which is available separately. The weight does not include the barrel nut as a result.
The length is measured from where it meets the receiver to the longest point of the front.
No, this will not work with piston gas systems.
I just measured mine, it's 1.75"
Just measured mine, it's 1.75"
10" is currently the smallest length available.