Crystal malt is said to positively influence head retention / foam stability in the final beer. Why is that?

The O.G. was 13 Brix (1.053), and the F.G. was 8 Brix (1.031). According to the calculation, the beer is only approximately 3% ABV. But I tried a glass and it's gotta be closer to 5.5%. 20% of the grain bill was Crystal Malt 10L. Due to the residual unfermentable sugars from the crystal malt, is it possible that the alcohol content is actually much higher than my calculation suggests? If so, is there a more accurate way to calculate …

Want to make a simple high gravity single hop ipa. Will this recipe work? 10 pounds DME Amber 1 pound Crystal 40l steeped 45 mins 1 oz Citra hops start 60 mins 1 oz Citra hops 45 mins 1 oz Citra hops 30 mins 1 oz Citra hops 15 mins 1 oz Citra hops 1 min 1 oz Citra hops off California Ale yeast I'm kind of iffy on how much Crystal malt is necessary to impart color or flavor …

What are the effects of changing the darkness of Crystal Malt? I'm an extract brewer, and many of the recipes I've tried call for steeping some crystal malt, but I'm not always able to find the °L called for. Usually I can get something close, but I'm curious what the effects would be of larger changes on the recipe. i.e. if I replace 25° crystal with 85° crystal (or vice versa), what will change in the final beer?

Does any grain/malt that's name begins with "Caramel" or "Crystal" mean it is able to be steeped to extract the aroma, color and flavor? I am referring to grain/malts like Carapils, Caramunich, Caravienne, Carahell, Crystal Rye, Crystal Wheat, etc... I am only able to do partial mashes with extracts at this time, but I don't want to be limited in styles I can produce and flavors I can get from specialty grains.