I forgot to ask - since the raised area has been there for so long...is it still a good idea to remove the structure....to maybe give it some room to recover?
Yes, it is. The root crown is a delicate area that needs to be exposed to the air. Many trees can exist for years with it covered to some extent and look "perfectly" healthy (with a full crown of green leaves); however, the toll it takes in the long run can shorten a tree's life significantly.
So I would take away the wood framing and carefully pull the dirt away from the trunk, feathering it out. What you will want to look for is a broadening at the base of the tree or the first signs of significant roots. You may see little adventitious roots and these are not to be worried about. Just spread the soil out evenly away from the tree.
Then I would suggest you provide an organic mulch ring around the tree (but not covering the freshly exposed root flair - not up against the trunk) approx 2 to 3" deep. Bring this out to the drip line if possible but at least a few feet away from the trunk. The feeder roots for trees are in the top 12" of soil (generally) and start a few feet away from the trunk. They are actually comparable to leaves in the fact that they die off and replace themselves regularly. So you need not, and in fact should not, keep the area right up against the trunk overly moist.
If you do want to plant flowers in this mulched area, choose plants compatible with the tree that have moisture requirements similar to the tree. However, planting too close to the trunk, again, is not in the tree's best interest.
With the garage being there for a long period the tree has probably already compensated for its intrusion. However, all these factors can add up as the tree grows and requires more space.
Sylvia