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The feature inside the red circle looks like a huge gash in the deck to me. It may just be the camera angle - perhaps you could photograph it from a different angle so I can make a different interpretation?

If the blade is not damaged, I don't see what would have caused the Mandrel Housing to have broken. It is a fairly major component, produced by expensive tooling, so it seems unlikely they are regularly snapping like carrots on all the Husqvarna mowers out there, especially since I haven't seen any specific complaints about it.

Here is a suggested approach to dismantling the the Mandrel Housing - you need the right tools, plus care and patience, and I suggest you stop whenever in doubt and come back here with some photographs rather than applying brute force. The aluminium housing is easily damaged (not too important, since it's broken anyway) and the shaft and bearings are not all that robust either.

First, you will need to remove the belt pulley (32), which is held on by a retaining nut (33), but the pulley is also likely to be a tight fit on the shaft (13). According to the illustration there is no drive key. I suggest you hold the blade (wear gloves and wrap the blade in thick cloth as well) while you undo the nut with a suitable ring spanner or socket. See if the pulley can then be pulled or levered upward off the shaft. You also need to remove the spacer (31). Having removed those parts from the top, remove the bolt (8) that holds the blade on the shaft, by holding the blade again and using a socket spanner. You are then ready to remove the shaft (13) from the mandrel housing (14) by placing the housing over something like a block of metal with the right sized hole in it to clear the outside of the oil seal at the bottom of the shaft, while providing good support all the way around the housing. Normally you would then apply downward force to the top of the shaft with a hydraulic press, but I'm guessing you don't have one. If so, you will need to place a piece of hardwood on the top, threaded end of the shaft while supporting the full circumference of the bottom of the housing on the block of metal with the hole in it. Tap the top of the piece of hardwood with a hammer, to drive the shaft downward. There will be moderate resistance to moving the shaft because the top bearing (15) will be a tight fit on it, and the bottom bearing (shown on the diagram but not identified separately) will be tight in the housing. The shaft needs to slide through the top bearing, and will probably take the bottom bearing out with it when it moves downward. It is just possible though that it will slide through the bottom bearing as well as the top bearing, leaving both of them in the housing when it comes out.

If you can remove the shaft and bearings from the broken housing without damage, and if the deck is undamaged, you may just need a replacement Housing, Mandrel from your Husqvarna dealer to get the mower back into action.