I wrongly thought that the Undo/Redo update to 6.3.0 was more extensive than it was, causing my undo/redo implementation to break, so I enabled IEUseLegacyUndoFunctionality. Instead, I found that only removing my ClearUndo() after calling Undo() has fixed this issue, as stated in the docs:
"In v6.2.1 and older versions, AutoRedo did not remove the undone item from the Undo List. From v6.3.0, AutoRedo clears the entries. To return to the older functionality, enable the IEUseLegacyUndoFunctionality define in ie.inc"
The compile error still may be something you want to look into.