I have seen one or two of those lower timing belt shrouds that were sliced by the idler roller like yours. I think it happens when the shroud gets installed and then the fan bearing/alternator bracket gets installed on top of it without getting the tab on the bottom of the bracket in the correct position. It is not obvious for someone seeing it for the first time that the tab on that bracket needs to fit on the inside of the sheetmetal belt shroud, not outside of it. If it is on the outside it pushes the shroud in to where it touches the roller. Easier to put the alternator bracket on first and then the shroud after.
Nice work on the tools and sounds like they did the job well. It was probably the lack of those tools when the last guy worked on it that caused the damage to the engine in the first place.
