Standalone = standalone engine management. Basically somehting that replaces the stock ECM completely.
An AFC is a piggyback fuel controller that modifies the inputs to the ECM in order to add or take away fuel. The AFC happens to use the MAP signal to do this. It increases the MAP signal to make the ECM add fuel, and decreases the MAP signal to dial in less fuel. If you used 450cc injectors, you would need considerably less injector pulse to idle and run them without being overly rich. If you used an AFC to accomplish this(-40% fuel trim), you would be lowering the MAP signal by 40% at the same time, which gives you enough room to boost up to 9lbs without throwing a MAP sensor code, since the signal to the ECM is within spec because it is being lowered by 40%.

With check valves, you set them up like this:

throttle body
[]
|_
|_
|_
|
[]MAP sensor

| = vacuum lines

_ = check valves

Basically short sections of vac line with tees connecting them. The check valves go on the ends of the tees to bleed boost, and the MAP sensor is still able to read vacuum.