My responses are based on what has been done the previous two years.
That's correct.
Not really. Check in begins much earlier than 6:45, usually by 6pm. Guests will be allowed into the party area at about 6:45 and the actual party portion should end by 8pm.
If SSG starts at 6:30 and if you're entering the party at 6:45, you'll be able to watch SSG before the party.
Another option is instead of going to the JBJB show viewing area at the end of the party, go watch SSG and return to the viewing area before the JBJB show starts. You'll miss the Movie Magic projection show, but IMO, that's ok.