Everywhere I've seen sweep defined is "how far back from straight", so 0 degrees would have the wing root and wing tips centered with each other, with any taper between root and tip equal, forward and back. 10 degrees back would be measured on the yaw axis, back along the wings center pitch axis -- 10 degrees back would have the wingtip's center at "(length * SIN(10))" behind the wing root center.
Sweep and stall speed is a complicated relationship. Effectively, the airflow across the wing at slower speeds bounces/drags across the leading edge causing it to flow *both* across the wing -- chordwise --, and along the wing -- spanwise. On a straight wing or a fast swept wing, it flows mainly across and most goes toward producing lift. the spanwise flow that comes at the slower speeds will reduce the lift, raising your stall speed. How much per degree . . . there are equations, but I've never had reason to fool with them, so I don't really know how complicated it really is.
There's also the issue with stalling near the tip -- there's more spanwise flow out at the tips -- adding on an unfriendly stall characteristic to the already higher stall.
You *could* mitigate this by installing wing fences to add drag on the spanwise flow, or washout to ensure the AoA of the tips prevent a stall before the root. Wing fences in general increase drag and can look cool or ugly, depending on taste, and washout only forces the stall speed to occur at the root, not decrease the stall speed. These mostly improve the handling, not as dramatically reduce the stall speed.
The best thing you can do to reduce the stall speed -- reduce the wing loading. keeping your wing loading low will allow you to maintain that 1:1 lift-to-weight with less and less airflow over the wing. Adding weight means you'll have to raise the AoA to maintain altitude at the same speed . . . and eventually your airflow will detach from the wing and into the stall you go.