What are the margins used to avoid a stall while slipping near the ground?
When I side slip my glider "correctly" the airspeed indicator drops to zero or at least shows large fluctuations between 30 to 90% of the actual airspeed. So the airspeed indicator essentially becomes worthless during a large side slip and can't give any estimate whether the aircraft is approaching a stall or not. And I wouldn't rely on it to judge if the aircraft is approaching a stall.
I can somewhat judge the speed from the sound of the wind and also how much rudder input creates what kind of side slip (slow usually creates more slip angle for full rudder deflection) and also the back pressure needed but the best indicator for the approaching stall is still the elevator buffeting or an angle of attack indicator or angle of attack warning. If you're planning on doing short field landings with low approach speeds then an angle of attack indicator is probably a good investment.

From a physical standpoint the indicated airspeed in a side slip condition should always be less than or equal to the airspeed that you're actually flying. So if you see an airspeed indication in a side slip configuration you're going to have at least that figure as actual airspeed. This is because the airspeed indicator just shows the difference between atmospheric pressure and total pressure (sum of dynamic and atmospheric pressure). And if you rotate your pitot tube from 0 degrees to 90 degrees off the direction of incoming air it will show less and less dynamic pressure and work more like the static port, thus the readout drops. But it can never show more than what the dynamic pressure actually is (disregarding small vibrations due to unstable inflow). So if you look at the airspeed indicator and judge by the speed it reads you should be on the safe side.

Whether or not the 10kt increment is enough depends on how steep your side slip is and what you are flying. Ground effect could also influence how much angle of attack is left before the stall. Only a numerical simulation would give you an exact number here.

But you can always experiment high up in the air, do a side slip down to what feels as low as you would want to go for a safe landing and then pull up even more and see how far you can go without it stalling or see how it departs (relax controls immediately to avoid any nasty spins) if it actually does anything. Maybe it will show some significant buffeting before it actually stalls (great indicator!), maybe it will just lower the nose (not ideal close to the ground but still safe) or maybe it drops a wing (not so good, add more safety margin).