The youtube scale RC 3 link has no pan hard that's why it moves around like that, add a pan hard and you eliminate that irregular movement all together.
As long as you build your lower links strong enough the upper link can mount to it without issue. One benefit of mounting it to the lower link is you can get away with using less material as the upper link will generally be shorter than if you mounted it to the frame.
I have a wristed rad arm set-up on my sas bii and under hard braking, like locking up the tires, I haven't noticed it dip to one side at all, she nose dives right to the bump stops and slides to a relatively straight stop in short order. Not saying yours will or will not, but mine doesn't exhibit any one sided dive anyways. Maybe because mine only has 5" up travel to the bump stops??
While pinion angle will change under full axle movement up and down with a rad arm type set-up, the changes while driving will be very minimal unless you air it out all the time at higher speeds and achieve full 2 feet of droop in the air, could make for a twitchy landing, but I doubt that will ever be your problem.
Having the 3rd link mount to the frame, you could make it the length it needs to be to keep castor angles through out full travel, but again, unless your airing it out at higher speeds that really is a none issue imo.
I'm not saying that you should go one way or the other. Just some thoughts I had on your question.