Ouch.

I think Volvo suspends the heater assembly in mid-air and builds the car around it.

If it were my car, I'd first try gluing the nipple back on the sump. What's to lose? If the experiment fails, you'll have to replace it -- which is where you now are.

Many of the plastics on our Volvo dissolves quickly when you use certain solvents. My favorite for this is methylene chloride, but I USE PLENTY OF VENTILATION. And it's highly flammable (what hydrocarbon solvents aren't?).

Hold the broken nipple back in place and properly aligned, so the fractured parts mate together seamlessly, and swap on some methylene chloride. It "wicks" into the crack, dissolves the plastic, and the plastic from both halves intermixes. It's a form of welding. Usually it's firm after 15-30 seconds and hard after an hour, or so (the solvent must migrate through the plastic to evaporate).

The repaired seam is about half as strong as the original.

I'd slobber on a healthy bead of RTV around the seam to seal up any pinholes or voids.

When all is hard 'n dry, carefully sneak the "Z" hose back on the nipple and pop the other end through the floor. As I recall, it slides up easily if you use a long screwdriver (done a bunch of them).
--
Don Foster (near Cape Cod, MA)