The breast collar you posted looks more like a 'tripping' collar than the typical packing breast collar, but I could be wrong. I've spent alot of time with packers, outfitters and guides and found that not only do they have alot of experiences, ideas, and mostly opinions, they also are more (exuse the the phrase, as no disrespect is intended) 'behind the times'. At least the old-timers are...
If you look at the skeletal make-up of a horse, you'll find that although breast collars are meant to keep the saddle forward they also inhibit the shoulder joints full range. When a breast collar is attached via the D ring on your rigging, the tugs come right across the shoulder joint; when attaching via a D ring on the front bars, the tugs also come across the shoulder joint but at a higher angle. However, when attaching via a D ring to the upper part of the saddle, just below the horn and swells, you'll see that there is no interference with the breast collar and shoulder joint (by using a Martingale type breast collar). But using this point of attachment will not allow the breast collar to do its job in a trail riding situation.
I wouldn't worry about interfering with the scapula too much, as the majority of it is up under your saddle, hence the 'withers'. And your saddle blanket will (or should) cover most of the spine of the scapula, so no worries there. Its mostly the neck and base and of course the joint where the scapula meets the humerus (shoulder joint) that will have most interaction with the tugs.
I myself would attach it 1st to the D ring on the front bars, and if there isn't a D there, I'd attach it (last resort) to the D ring on the rigging. But not if its dropped rigging... I don't think your mod. association has dropped rigging though...? I agree, attaching to the riggin D is sloppy and used more by ppl who don't know any better. I'd hate to have those tugs flopping around and run a stick through there and find yourself in a wreck... In the hills, using the martingale type would serve no purpose.
If you were to ask an "old time" outfitter he'd probably tell you to use the packing breast collar, as thats what he's used and its done him no harm. And he probably has 6 spares hanging in the tack shed! But that doesn't make it right.
To me I'd rather have the wider leather, more typical breast collar. In my opinion they are more ergonomically correct (if I can use that term), and don't hinder the shoulder joint as much as a typical packing collar. When going up steep hills etc I'd like my saddle horse to have a freer range of motion, and as my saddle horse I'd probably be putting him in different situations/paths than the pack horses. If I need to get infront of the pack string, or get out off the trail I'd like the reassurace of knowing that a) my breast collar tugs are not flopping around and b> my breast collar is serving its purpose of keeping my saddle forward so I'm not sliding back on my horse and soring him.
If the breast collar is done up properly, there shouldn't be any need for a hobble up and over the neck. But riding different horses will tell you if you need one or not...
You'll also notice that pack horses not only have the breast collar in the front they also have the britchin in the back. This helps keep the load centered. Enter the rear cinch....
That being said, its just as important to have your rear cinch in the proper postion and SUCKED UP! Don't be afraid to pull the cinches in him. You won't hurt him and it'll keep you saddle from sliding back and forth. You rear cinch is there for a reason - USE IT! Thats probably the biggest problem with ppl in the hills, and could very well be one of the most important - the rear cinch. Its there for a reason, use it and use it properly, and you'll find your horse won't get sore from your saddle moving or from a sore back, both caused by improper saddle position.
Help your breast collar out by using your rear cinch - your horse will thank you for it after a long day in the hills!
Sorry to be so long winded Hope I helped a little...