I agree; the only way it makes sense is if the third-party non-alliance-member will do it on more favourable commercial terms and/or with more appropriately-sized aircraft (so the big carrier doesn't have to carry fixed costs and commercial risk, or fly aircraft that are larger than ideal for the route).
Absolutely right, and in the current climate, even the big three legacy carriers in Europe cannot ignore the this. There is precident for it too after all, it looks like when BMI Regional come off NCL/BRSBRU, despite having Star Alliance equipment of the right size within the likes of Eurowings and Tyrolean etc, these routes will be operated by Flybe.
I guess the Q400 gives increased capacity at similar or lower operating costs to the E145 and made Flybe the better choice under the circumstances.