The interest isn't really going to make much any difference in the situation you have outlined.
She has very low income so isn't able to utilise the Personal Savings Allowance but if her casual jobs and taxable pension withdrawals just use up her Personal Allowance then the interest would all fall into the savings starter rate of tax which is currently 0% (a maximum of £5000 can be taxed like this but the amount reduces as wages or pension income exceeds her Personal Allowance, once it is £5,000 over then she would have to rely on the Personal Savings Allowance rate band (also 0% but only for a maximum of £1,000).
So under your plan the interest would probably be fully taxed but no tax to actually pay on it
Going off the threads on here contributing to a pension in this way is extremely popular, particularly when the expectation is that no tax will be payable when the taxable element is taken.
The only thing I would question is if your basic figures add up.
When she is 55 she will have about £30K
Assuming your reference to taking out as much possible means to make her earnings/pension income up to the level of her Personal Allowance then i'm still not convinced that £30k plus ongoing contributions will be sufficient to do this for 12 years.
How have you calculated this?