Best answer is to follow the manufacturer's instructions! Basically, you are doing two things by supporting the bottom of the tub: making it feel more substantial and preventing stress fractures in the floor when it flexes slightly when you step in. The foam will help, but it is flexible on its own right.
Standard plumbing in the US has the hot supply on the left and cold on the right. Each tub/shower filler will have its own offset and setback, there is no real standard. That is how far apart the supply lines are and how far back from the front of the finished wall it needs to be so that the trim fits properly. Most supplies use 1/2" inputs, but some are 3/4". The larger ones are often used for really large tubs or multiple head shower systems so that they can flow more water in a short time. Unless your house is unusual it probably has 1/2" lines going to the valve. 3/4" happens, but is not the norm although it may not be far away. Any new shower control must be either/or or and a pressure balanced or thermostatic design to help prevent scalding if say someone flushes a toilet and suddenly takes all of the cold water for instance.