How to avoid using door trim
I'm going to take a stab at this one, though I've never tried this for something large like a door frame.

Go get some self-adhesive fiberglass drywall tape and cover the area to be blended. Mud the seam, sand and repeat until smooth and the surface is blended with the wall.  Prime and paint to match.

Alternatively, you could try peeling the paper off some scrap drywall and gluing it to the wall, and then doing the same as above with drywall compound.

I think these will help prevent it from cracking.  You'll want to make sure the door frame is well secured to minimize any "give" that would crack it.