How do I avoid hot fudge turning my ice cream into a soupy mess?
I don't have any direct experience to share, but it seems a little logic may be applicable. I suggest that your hot fudge is too hot and your ice cream is not icy (cold) enough.

Rather than microwaving the fudge, try a hot water bath on the stove. Yeah, it's slower, but you also won't burn the bejeezus out of the sugar at the edges. Taste occasionally while stirring until you get that pleasingly warm mouthfeel. Stirring frequently* will help distribute the heat.

Alternately, microwave in small bursts, stirring* as you stop and taste.

*Sanitary practice compels me to advise you to use a fresh spoon every time you stir and taste in order to avoid contamination. But it's your house.

The hot part being taken care of, make sure your freezer is actually freezing cold. Don't take the ice cream out ahead of serving so it doesn't soften. Best way to scoop hard ice cream is to use a metal scoop dipped in very hot water. Cuts through like buttah.