[blue62]

After getting more information from the OP in post #22 where he describes how the car reacts to throttle. And also the fact that it is backfiring into the intake.
Here is what I think is happening.

There is some issue with at least one cylinders valves.
OP names #4 cylinder.
There is some issue with the intake valve. it is not sealing at the seat.
Due to something like a broken spring. a weak spring. a collapsed lifter.
A sunken or damage valve seat. Something is preventing that valve from sealing properly.
The only way a backfire can happen in the intake is for burning fuel to get past the intake valve.
So I think there is an intake valve issue.

If there is a valve issue the fuel/air mixture is not burning properly in that cylinder.
So exhaust from that cylinder is rich in oxygen.
So when the exhaust from that cylinder get to the O2 sensor the O2 sensor sees only the oxygen. that's all it reads. So O2 sensor voltage goes low.
The DME/ECU sees this as a lean condition and adds fuel.
Now you have a very rich condition on that cylinder.
If there is a valve issue with more then one cylinder the same thing is going to happen.
The more misfires the more fuel is added by the DME/ECU

IF I am correct then the spark plug on #4 cylinder will smell of gas.
Or any cylinder that is misfiring
This is also why the OP smells fuel on start up.
Most of his misfires are at 1500 RPM or below
Also if a compression test is done #4 cylinder it will show lower compression then the other cylinders.

Now having said (typed) all this I could be wrong.
But I can only think of one way to get a backfire into the intake.
"Intake valve issue"

OP may also have injector issues. (Leaking)