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You need a vapor barrier to stop water vapor from entering the insulation and condensing when it gets to the cold side of the insulation. Yes doubling the thickness doubles the R-value. Climbing on the insulation can be avoided by making walkways above the insulation. Of course dealing with a roof leak will be complicated the wetting of the insulation. However the wetting should be localized to the area of the leak, and heating and low outside humidity will dry it out. Fibergalss is glass, and glass does not soak up water. Gravity will pull the leaked water down, at which point you will discover the leak.

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