We also say that this is a good time to write the story because we now have time to help them. We always give them time to finish the drawing later. I teach first grade and I have concerns about the children going to 2nd grade with teachers who don't embrace this technique. I'm not sure that I understand your concerns. If you believe in your teaching practice, this is not an issue.
Children can learn quickly how to do phonics pages.
I really need more information to answer this question completely. If kids have done lots of kid writing, they will be very proficient at high frequency words and will have very thoughtful phonetic spellings for words not yet taught. They should be further along in their writing than kids who have not written daily and have not had mini-lessons addressing both the conventions and the craft of writing. The children know that for every spoken word, something has to be written down. In using the magic line, the children are empowered to write their thoughts using vocabulary in their oral vocabularies, not just words they know how to spell.