Any particular reason you're looking to change out your crankset? Gearing change? Weight? Aesthetics?

The reason I'm asking is that your chainrings are bolted to the crank arm, meaning you can swap them out without having to change the whole crankset. You may be able to accomplish what you want without buying a crankset or bottom bracket.

There's nothing that really makes a crankset a "touring crankset" other than chainring selection -- basically making sure your small chainring gives you low enough gearing to get over whatever hills or mountains you might have to cross. Thus, "touring cranksets" are typically triples with a small (30-tooth or less) inner chainring.

Triple "mountain bike cranksets" can often be found with small inner chainrings ...and they can make fine touring cranksets.

Sure, you can use two-out-of-three rings on a triple, replacing the outer ring with a guard -- a "bash guard" in mountain bike parlance. You're right that you'd want to set the limit screw such that your derailleur can't shift up and onto the outer ring. That's a perfectly functional setup. The only oddity that you might notice is that you won't be able to shift into the "3" position on your front (left) shifter.

47.5 to 50 mm, measured from the bike's centerline to the middle ring, is textbook for a Shimano mountain triple setup. (50 gives a little extra clearance for the derailleur to swing if your frame has a large-diameter seat tube.) 45 mm is the spec for road triples.

Your math is right, but this is one of those instances I might be tempted to obtain just the crankset and test fit it on your current bottom bracket. If it works, great! If not, it's an easy measurement to figure out what bottom bracket spindle length you need.

As for what's correct... My guess, based on Stronglight's own web site, is that a 115 mm spindle will give you a 45 mm chainline. Stronglight's site specifies a 115 mm spindle but doesn't explicitly list chainline. Since it's a road crankset, though, and 45 mm is typical chainline for triples in a Shimano road bike drivetrain, I think it's a reasonable inference. The chart on Sheldon's page (which may have been sourced from an authoritative source) seems to agree.

As for what you're missing... What chainring sizes do you intend to run? The derailleur may have to be repositioned to accommodate. If the chainring sizes are significantly different than the inner two rings on the original, the sculpting on the derailleur cage that's intended to aid shifting might not line up quite right. (Probably not a huge deal and may not be an issue at all... just trying to dot all the i's and cross all the t's.)