How to interpret this data sheet?
Nothing wrong about the spec as far as I can see - it sounds like you want it couched in terms that are meaningful to your limited knowledge and application requirements.


1 Mohm minimum input impedance is fine - it's 1 Mohm guaranteed or
greater - most folk want high input impedances so a guarantee is a
good thing.
Common mode input voltage range applies to differential amplifiers
and it basically means, if you want to measure a small AC signal (say
100 mVp-p) it will do so even if both input wires are raised by a
potentially interfering voltage of +/- 10V. Good spec I reckon.
Common mode rejection - this tells you how good the input amplifier
is when it is connected to a source voltage with different resistance
connections. 70 dB at 60 kHz is pretty good for this sort of thing -
you have got to envisage that some folk may not be able to adequately
control the impedances of their signals that are to be measured by this device.
Input imbalance adjustment - no amplifier is perfect and most will
produce a small dc error at their output - this can be nulled-out by
the adjuster.
Linear output is telling you that it won't produce significant
distortion (maybe due to clipping) for output peaks up to 10V. I also read this
as meaning that the nominal rated full scale output level is +/-10V.
Corner frequency is the point where the output starts to naturally
fall when the input frequency gets above a certain value and for the
default amp you are using this is 10kHz. It's adjustable.
Cross talk of 80 dB RTI - referred to input is what RTI means. Cross 
talk applies to amplifiers with multiple channels and interference
between one channel's signal and another channel.
Gain resolution - if you want to adjust the gain there is a finite limit to how exact you may want it and this is the resolution.