If it is not blocked under the sink, the block must be after the sink piping and in the downstream section of the drain line. Have you snaked this line? Is the drain line vented properly?
I wouldn't go that far. It may not be the problem now (this time), but the excessive pitch makes any vent pipe downstream useless, and Fernco couplings are not a good choice in any long term installation that could've just as easily been glued.At least you know your work was fine.
Before paying somebody $125 per hour to auger your drains, go buy one of these drain cleaning bladders. I've got 2 different sizes, and cannot begin to count how many times I've used them to blow out plugged drains.well i did some more testing last night, it is a problem downstream of the stuff in the picture, i found an access cleanout thing in the basement directly below this sink, snaked it 25 feet, (was a plug at about a foot and a half)... and it still didn't clear,
i think the pipes in the basement are in bad shape, seemed like there was crud all along, i think i need to hire someone to look at it