I personally use everything in the vial (no wastage, it's $$$) and if it's not enough for the injection, say 3/4 of what I should be injecting, then I inject 1/4 of the new vial in the other butt so that if, ever, I get a reaction, at least I know which vial caused the reaction. I wouldn't ever mix both and inject in same area.
For Estradiol Valerate, it seems most do well on a weekly basis although a few do ok bi-weekly or every 10 days, I personally take it every 5 days. For Estradiol cypionate, it can be 10-14 days.
I sometimes get sore, sometimes not. It could well be due to several factors like the excipients in the vial, how tense the muscle was at the moment of injection, how the nurse injected, where exactly (higher above the butt hurts less than outer quadrant of the butt where there are more nerves).