Without taking the wardrobe out of the alcove you are f*****d as even destroying the shelf to remove it will still leave you with the problem of the dowels reinserted in new holes not letting you secure the shelf. If you insist on doing it in situ then you will need to use an alternative method of securing the shelf such as battens screwed to the sides with the shelf resting and secured to the battens too for the reasons mentioned in my previous post.

Richard