I do my P07/P09/C75/P01 Omega, all of them that same way.
Empty it, remove the magazine, move the hammer to halfcock, grab in with my left hand with my thumb over the backstrap and fingers over the top of the slide and squeeze and the slide moves to the rear and contacts the half cocked hammer and use a magazine corner to push the slide stop towards the left side of the frame.
Once it moves over, switch hands to the right, squeeze in the same manner and use my left thumb and trigger finger to grab/pull/wiggle the slide stop out. Works great.
I never even look at the witness marks because when the hammer is at half cock and the slide is rear enough to contact the hammer, the marks are lined up close enough for the slide stop to loosen/revmove or reinstall.
Yeah, use the right hand grip to squeeze/hold the slide rearward against the hammer at half cock and the left hand to reinserted the slide stop. Once it's as far in as I can push it with my thumb the plastic/rubber base on my magazine will push it the rest of the way. I really don't have any magazines with metal bases anymore. They've been converted to the bases that don't scratch stuff when my big wide rear end walks past with the pistol hanging off the right side.
