Stay off that slope until its safe since you do not HAVE to ride down it.
You might have 50 years experience, but you might just be doing the same thing for 50 years. That is not 50 years of LEARNING experience.
I do applaud that you keep trying to educate yourself by reading books, but you seem to decide on a fact and then find a book to back it up. Since you use books from a spam of 100 years and my all sorts of people, its likely if you look hard enough you will find one that agrees with you.
The standard modern approach is to stay in the same position for a hill–not a steep sliding almost vertical drop, as you would for the flat but to bridge your reins, grab neck, do something to keep you from falling or sliding forward and get out of the horses way. If you are walking then you do not get into 2 point and assume a forward out of the saddle seat. if you are at speed you do for the same reasons you do it not on a hill.
But once the angle of the hill requires you to change your position in order not interfere with the horse then you must sit up, stay light and do you best to leave the horse alone so it can get down safely. Not load either end, not dump more weight on its forehand and not lean back hoping to change the laws of physics by imagining your weight has shifted to the horses rump because your head is near in.
The picture does not show the position for riding down a steep hill. The pictures shows the position for 1-2 strides down the hill and then a jump the rest of the way, so yes, a jumping position would be best. But you would not worry about footing our anything else since you would be airborne.
All other pictures and manuals of modern times say to keep yourself in the same position as the trees. You do NOT lean back. The horse goes down from underneath you in the front. It just looks like leaning back. Just like you do not duck down over jumps, but allow the horse to come up and meet you as you stay quiet and still in your 2 point position. It LOOKS like you have to duck down–but you don’t. The horse rises up to meet you.
But once again, when faced with a solid sheet of ice on a very very steep slope the correct answer is to apply some reason and go another way unless it was some matter of life or death. If you MUST go down, just get off. Because its not a movie and there are no special effects. Good horsemanship is not like stunt riding. Its planning and avoiding unnecessary risks and doing your best not to waste a Medi-vac teams time.
Just like in that clip from man from snowy river, the horses with no riders have no problem at all. They do not need your help. They need you to not hinder them in any way.
You keep telling us you are older but guess what!? I’m older than you and been riding for sixty two years. Done lots of stuff and lots of hills (still do) and not one time have I ever “wiped out” on a hill. If I were flipping horses backwards on down hills I’d reallly question my riding or my horse. That is weird. Having said all that I have to say even at my advanced age :lol: I still am learning new things about horses, riding. You are truely missing out on one of life’s pleasures because you “know everything.”