I should have analyzed the patterns before started to solve, which would have made me find one solution faster.Nice analysis in that post. I kind of did the same analysis when I tried to solve the 9-color cube with one difference: I didn´t come up with a solution before even solving the puzzle, I was figuring out (gradually eliminating) the solution while solving the puzzle. The consequence is that my solution will lead to a different solved state each time.
Definitely corners-fast method is the option to take in this puzzle. I can say it now, because I used that method to get the 80 possible patterns.I did the corners-first solution, my idea being that if corners are solvable, the edges would be solvable as well. When the white center is facing down, the white corner belongs to the top layer. Green corner belongs to the top layer as well. Similarly, orange and blue corners belong to the top layer as well, so I made these corners solvable by placing them "correctly". Next I made the corners in the bottom layer solvable, so that all corners are in a solvable state.
Hard work!Next I tried to solve, one by one, the edges from your group C (edge-center-corner group). I indeed encoutered a situation in which 2 edges required to be placed in one place, realizing I need to find another solvable state for corners - so I cycled the corners (green corner not included in this cycle) in one layer so that the 8 corners would be still solvable relative to the centers. Eventually, that lead to the corners and all except red and magenta edges being solvable.
Bad luck, The probability to enter into that situation is 20%.As my last step, I tried to solve the group B pieces (red and magenta edges). I indeed encoutered a situation in which these edges are not solvable, realizing I need to make an E or E´ move. Only one of these moves will make the corners still solvable, so I went with that move. Next I solved the edges from group C again, and finally, group B pieces would be solvable, thus solving the whole puzzle.