Does frequency &leakage have anything to do with back problems between l3-s1?

Yes! You are absolutely right! that certainly could be the cause but they are also other causes that should be considered. Visiting a urogynecologist or a female urologist is your best option in diagnosing and treating your condition.
Yes it can. Often if the nerve roots are compressed or irritated bladder leakage can be a problem. If you lose control it can be an emergency and you should seek medical care immediately.