1. Best bet is to look at completed builds and get a feel for common practice. Double checking your wall thickness and chamber volume to muzzle pressure is a good idea, just to be safe.
2. I'm not familiar with any of those. Just make sure it's bored for your caliber and pitch matches barrel, in that order.
3. You could use any metal really. I prefer to mix it up. SS closer to muzzle and Ti or Al for the rest.
4. I've only ever not clipped 1 build. It was for a rifle averaging 600+yd shots. I was worried about POI and so I left them alone. So, I clip everything (now).
You don't need a design to start a form1. Caliber and length is it. Say 450, 458, 50cal whatever, and you could put 20" if you want. When you get approved and finish building, just send ATF a letter with the amended length (being shorter than 20"). 20" is just an exaggerated example. You can put 11" and build to 10.5", if you feel more comfortable.