Learning to fly with clipped wings is like learning to run with your shoes tied together.
But yes, clipped birds can and do fly away all the time.
Smart Selects has too much seed for a Quaker, especially a clipped one, in my opinion. Quakers are prone to fatty liver disease, which is encouraged by a fatty diet and low exercise. I didn't allow my quaker more than 1/2 tsp of seed a day, and only in foraging toys.
There are plenty of pellets available:
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Before breeding your birds need to be on a healthy diet for at least 6 months, and really should be at least 2 years old and unclipped. Flying actually strengthens the muscles needed for egg laying, so a bird that is clipped and on a poor diet (especially those low in Vitamin A, D3, and calcium, which seed heavy diets are) is at serious risk for egg binding.
The younger the birds, the higher the risk for complications and chick abandonment as well. Even then, some birds make terrible parents and you'll have to intervene regardless.
This all assumes they even want to breed. My male quaker lives with my mother in law with a female Quaker. She lays eggs, and he masturbates, but they do not have a pair bond and do not mate with each other, and if they were housed together, they would hurt each other.