The LinkedIn summary= gets chopped down content instead of the summary field. If the LinkedIn summary field has a special limitation/requirement on length, the best approach would be to add a field to the content for the LinkedIn summary so we can edit the summary to the right length. The second best approach would be to use the existing summary field and add a note under the field about the limitations of the LinkedIn summary. A random length of text chopped from the front of the content rarely works.
The first option would be good for your planned version 3. Adding a reminder to the summary edit box should be an easy option for version 2.