You did not say in your first post if anyone actually rode her in the backyard.
Sounds like a carnival/pony ride pony. They are very loving and will spend all day doing nothing, but haven't actually been trained to ride. Yes you can tack/mount and they move off without any bad/dangerous behaviors. But have spent most of their life being lead or tied to a walker with kids on them.
Best case if you and your trainer can, is just start over from ground 0. As if she was unbroke, the parts that were done correctly should go fairly easily. The spots that are lacking will take a little extra time.
Good Luck