Re: Balanced/Inbalanced field length
No, 738 jockey, you don't unbalance the field. Being a balanced field is simply a characteristic of the runway, the ASDA(accelerate stop distance available) being equal to the TODA(take off distance available). Which is another way of saying that stopway and clearway are equal.
If you are not performance limited, you can, as you say, use a range of V1 speeds, but that does not change ASDA or TODA. It just means you don't need all the available length - which always is a comfort. When you are performance limited, there is no room for error - so that decision at V1 is really critical on those occasions.