1. Are you sure a spatial arrangement is indicated? If so, there is no rule stating a minimum number of cells for a separation line, so a 1-cell line is fine.
2. Volume size is defined by the agency for whom you work. I am not aware of any recommended minimum count for a volume. Maximum sizes are stated in Braille Formats (Rule 1, Section 9) or as dictated by your production agency. Grade level, binding method, and paper thickness are among the considerations for establishing a maximum page count. Braille Formats suggests that braille volumes end at a logical break in thought, e.g., at the end of a unit, part, chapter, or section, and that "adherence to this principle is more important than maintaining uniform volume size throughout the braille edition."
3. In Nemeth Code, the "con" contraction is not in the list of restrictions for contractions in contact with a grouping sign. Yes, "con" should be contracted in (cont.) and yes, although it could be misread as (3t[sub]4) context should make it clear that it is to be read in a literary manner. There are few ambiguous constructions in this code, but they do exist.