First off, SSD GOL (Swedish Stump Dance/Goon of Logging) has its place, but its a narrow sighted ridged set of rules for falling timber, while some of it makes sense, other parts of it will get you in trouble, trees fight back and don't care about what ever rules you learned in your weekend logger course. After all the trees don't read Swedish or English, and are belligerent and vengeful ********, bent on the destruction of all of mans works.
From what I understand Ash is just a PITA chair prone tree no matter what you do, dead and dry? does its split easier green or seasoned? the equivalent we have out here is Alder, which is most dangerous when green and the sap is running, it hardens up a lot when dead standing, though it can still kill you.
As for critiquing your stumps, face cut to shallow i.e. not deep enough which on a leaner means you have to blast through that much more wood before it tips giving the tree that much more time to stall, and then chair, granted you don't want to go as deep as a straight tree, but considerably more then ya did, then consider boring and leaving a safety strap, (as GOL teaches to do on every ****ing tree) or better, bore the guts of the hold wood, leaving wood on each side, though on this one, if you had cut a little deeper you would of been well into the rotten core, and not needed to...
Now, as for leaving safety straps, I rarely do so, but will if its super sketchy, the benefit is it gives you a head start on vacating the area, however, your other cuts have to be correct or it can still chair, or worse you over cut the hold wood and the tree crushes it and pinches your saw (this realllllllllyyyyyyy sucks btw) which is bad in all sorts of ways, not just pinching the bar but it also compromises the integrity of the hold wood....
Anyway, next time take a little more on the face... it will solve most of the issues.