My question is , " how does the 100 ohm resistor affect the vr2 potentiometer of 0-100k ohms. I believe that we treat it as a resistor divider as 100/(0-100k) times the dc bias of 0-5volts. If this is true, why use such a large value potentiometer. Anything over 5k ohms would have little affect on the resistive divider?

 

Not if the difference between the 2 100-Ohm Resistors is
amplified in some other part of the Circuit which is not Shown.

If You would provide the complete Schematic, the reason could possibly become obvious.
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It looks like the pot is there to tweak something over a small range, perhaps as residual offset compensation.

Also, many opamps do not like driving capacitive loads and it is pretty common to see series resistances in the 100 Ω range on the outputs in order to keep them from breaking into oscillation.

 

Not if the difference between the 2 100-Ohm Resistors is
amplified in some other part of the Circuit which is not Shown.

If You would provide the complete Schematic, the reason could possibly become obvious.
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Re: U5 .........
Just scan over the Data-Sheet .......

"" Care should be taken to ensure that the output impedance of the sources driving the +IN and –IN inputs are matched.
If this is not observed, the two inputs could have different settling times.
This may result in an offset error, gain error, and linearity error which change with temperature and input voltage ""
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The 100 ohm resistors are not there to affect the effect ofthe 100K ohm pot. They are there too isolate the op-amps. Post #3 mentions that.