It's not at all unusual that a hotter load is more accurate than a milder flavor. Guns are like children, each with their own personality.

That said, the quickest way to come to grips with revolver accuracy is to measure your weapon. Knowing the dimensions of your cylinder and bore will give you instant insight into how a particular bullet is likely to perform... before the first round heads downrange.

Yes, a steady diet of hot .44 Magnum loads will loosen a Model 29. Probably more quickly than you'd like. There's much to be said for feeding it a balanced diet!

There's no reason not to shoot your big N-frame directly off bags. But I'd suggest you extend your distance. Careful technique and a hundred yards will quickly tell you what you need to know about any given load.