This is not a free float rail.
The Guntec USA AR-15 Carbine Length M-LOK Handguard w/Monolithic Top Rail will most likely not work with a gas piston system.
Yes, the Guntec USA AR-15 Carbine Length M-LOK Handguard w/Monolithic Top Rail will fit a Ruger AR556 with a A2 Front Sight. This is a two piece handguard so when installing it you wont need to slide it over the barrel and front sight.
The Guntec USA AR-15 Carbine Length M-LOK Handguard w/Monolithic Top Rail does not have to be installed at the same time. Press down on the delta ring and install the top half of the handguard. Then press down on the delta ring again and install the lower half. You can use an AR-15 Delta Ring Tool to assist in installing.
https://www.opticsplanet.com/wheeler-delta-ar-15-delta-ring-tool.html
Yes, they will fit the S&W MP 15 carbine
Yes, there are 3 M-Lok slots at the 6 O'clock position. Your plan sounds solid.
Yes, you can mount a light, or many other attachments to this handguard.
Yes, this would work.
Yes, this will be compatible.
That's correct, this is a two-piece drop-in handguard for a carbine length AR15 with a delta ring and end cap.