Just got back on this project. I had a professional plumber assess the situation. It was determined that the original flange was sitting too high anyway, so it was removed. After setting in a flange that went on the outside of the 4" sewer line, we still weren't able to buy enough space to prevent the back of the tank from touching the wall (we're dealing with fractions at this point).
Although I've already invested in the Toto Ultramax II, begrudgingly I think getting the Toto Drake with 10" rough-in is the wisest approach, as it eliminates both the risk and cost factors involved with cutting the tile, digging, and then attaching the offset flange's stainless steel ring to the tile surface (which I fear may cause the tile to crack). Not to mention how based on how the current tile is cut, the ring won't be able to sit on tile on all sides, so might not be rigid enough without adding support below - although I'm not sure this is a problem.
Question: how far off the wall should I anticipate the 10" rough-in model to sit, considering I'm at 11"? A little concerned, as I read a review where someone managed to fit this model in when they had a 9" rough-in.
Edit: another decent 10" option might be the Kohler Highline Classic. If it will sit closer to the wall, it might be worth considering.
Thanks all for your help and assistance!