How to prevent strawberries from discolouring cream

I don't think this has a single answer: The amount of alcohol would depend on the alcohol content of the soaking liquor or liqueur, soaking time, temperature of chocolate, and how thick the strawberry was.

I suspect uncut strawberries would take at least several days to fully absorb alcohol and reach equilibrium, but the surface should take up alcohol fairly quickly. If they are cut up, exposing the more porous interior, I imagine an overnight soak would be sufficient to reach maximum alcohol content. The enormous, bloated mutant strawberries you sometimes see at stores could take considerably longer than others to absorb their maximum alcohol.

Alcohol is much more volatile than water, so the immersion in warm, melted chocolate would remove some alcohol, but once the chocolate cools, it should trap any remaining content.

The flavor of alcohol is easily masked by other flavors at under 20% content; this could explain why your friend didn't taste it, but you did. It's quite possible the strawberries packed a sobriety-busting punch, but it was disguised by the chocolate and fruit tastes.

You could replace the sugar with some artificial sweetener. I don't know the exact ratio, but I believe you can use less sweetener to an equivalent amount of sugar. But by definition syrup contains sugar, so you wouldn't be able to eliminate it (or a replacement completely).

I suppose, depending on what you're doing with it (topping for dessert, waffles, pancakes), you could just take the strawberries, add some water and lemon juice, and reduce it and make a sauce instead of a true syrup...