Welcome to AO, and congrats on the Minimag. That's a great starting point, especially with the Benchmark frame.
Let's see about your questions.
Most mag frames are plug and play, especially the ones straight from AGD. A few of the aftermarket frames (like the Logic frame, for instance) don't fit the shorter rails, but they are few and far between.
No. There's a significant difference between the old classic AM/MM length rails and the newer RT length rails. The older, shorter rails only have one mounting hole in the front, while most of the newer rails have two mounting holes (or they have a groove for screws). You might be able to still mount the foregrip with only one screw, but then it could spin if it's not super, super tight.
(The old style foregrips got around this problem by having
"wings" on the top that would hug the angled bottom of the rail and keep it from spinning.)
This is a matter of opinion. Personally I think that a double trigger (which you already have) makes a bigger difference than an RT on/off. But having both certainly doesn't hurt.
You can drop an x-valve right into your Minimag. Shouldn't be a problem.
Good luck, and have fun.
Oh, and if the gun has been sitting for a while, you should order a parts kit from AGD when you get the RT on/off. You will probably need to replace all the seals sooner than later.