Pretty much, from what you have stated, it does sound like the injector had a problem and the problems went down the line.
Overfueling will destroy a lot of stuff.
The crack in the manifold will cause the O2 sensor to read too much air and give a fault. It will also change the air/fuel mixture, which could cause the fuel metering fault. The problem comes from when the computer tries to compensate for the crack and it may over fuel, which is the problem that started all this in the first place.
Unfortunately, you are at a point where the cat, exhaust manifold problem and the O2 sensor need to be taken care of.
You have to fix the known problems first, then deal with what you can't see.