I've been loading for the .357 mag. for about 30 or so years, and have never had anything even remotely simular happen. I've never used Unique either, so my perspective may be off a bit.
Then there is the blown primers. It may sound stupid to ask this, but are the primers extremely flattened, or just punched? I'll bet both?
Double check the powder weight being used, the bullet weight confirmed, and even confirm the powder to be sure you haven't inadvertantly loaded the wrong one, it happens. I wouldn't even consider such a possibility if not for the fact that your experiencing such extreme circumstances.
Speer data shows a 158 gr. SWC to have a charge range from 5.5 grs. to 6.0 grains with Unique. Maybe your powder charge is too high for that plated bullet. I've heard many suggest starting with lead data for plated bullets, and then working up slowly until your where you want it to perform, or signs indicate the maximum has been reached. In this respect, I noticed your powder charge exceeds Speer data by a good bit, .8 grains above the maximum. One thing for certain, I would not consider increasing the charge with the signs your experiencing at 6.8 grains, especially with a magnum primer.
And last, but not least is the primer you are using. You said your using a magnum primer. Unique doesn't require a magnum primer, and using one will most deffinitly increase pressures, especially with fast burning powders. Magnum primers are only necessary with powders that are difficult to ignite like ball powders or large dense compressed powder charges when it is recomended, certainly not for Unique.
My suggestions is to use a standard small pistol primer, decrease your powder charge by .8 grain, and make sure your OAL is not too short. I'll bet if you try this you'll stop blowing primers at the very least, and even possibly eliminate the tumbling bullet problem. Once you've stopped the primer issue and tumbling bullets you can slowly work up your powder charge in .1 gr. increments, if desired.
Good luck and be safe!