Why can't we just assign m = m_start?

The vm_object_populate() call above guarantees that the object has resident pages for pindices [pindex, pindex+size). I don't think we can have p == NULL here.

Why is it ok to drop the physical address check?

That won't set pages.index to m_start->pindex, so iter_step() and iter_jump() will be adding to an incorrect or uninitialized value to update the index value.

Because line 7940 is
p = vm_radix_iter_next(&pages);
and not
p = vm_radix_iter_step(&pages);

The latter would be equivalent to "p = TAILQ_NEXT(p, listq). The former gets the page with the next address, and gets NULL if there is no such page.

Contiguity in the pindex space doesn't imply contiguity in the physical address space. The vm_object_populate() call ensures that a page is resident at all pindices in the range, so the old code could have equivalently used vm_page_next().