At this page you can find the microphones polar pattern and frequency response (the ones you provided), so you could have possibly found out before buying that the polar pattern is not a "perfect" cardioid.
Now, regarding to whether this polar pattern should be called or not a cardioid is somewhat debatable. Personally, I would call that hyper-...
The audio codec is not really relevant.
If capturing noise profiles does not work, then you can try operating with low and high pass filters. High pass filters help to elimiate humming of car motors. Low pass filters help to reduce high pitch noises, like screeching brakes. Of course you have to be aware not to cut off relevant frequencies from the vocals. ...
These specs don't tell you in much detail how the mic will sound. A full frequency response plot gets a bit closer, but it remains difficult to compare audio devices using just their specs.
One thing that jumps out from the specs is the massive difference in sensitivity. This makes me think the specs either can't be compared directly (because they have been ...
You cannot choose based on those numbers alone.
Though 50Hz - 15kHz might seem a little limited, if all you are recording is spoken word, then it's sufficient. The values alone don't give any indication as to how flat the pickup is across the audio spectrum.
The dB sensitivity figures have no proper units so cannot be compared at all.
You have no way of ...
Once you record, the achievable signal to noise ratio becomes fixed. So, if you increase the volume on playback you also increase the noise. In general it is better to increase the gain of the mic pre so the recorded level is -18dBFS in order to maximise the signal to noise ratio.
Gain is more technical and can be discussed and measured in specific terms. Volume isn't really an appropriate term in mixing. It relates more to final amplification. There's no good way to relate one volume level to another. Sound pressure level is what's usually meant when people speak of volume.
Without being able to hear the sound file, I'm going to assume your recording has taken the usual input via mic, plus the same again through some internal or external route very slightly delayed.
If that is the case then no, there is no way to fix it. Like most issues where you have additional voices, you'll have to re-record.