I’ve never done the Hungry Bear package so I don’t know all the answers to your questions on that package, but here is a link to a Disneyland expert blogger who can provide a lot of answers for you: Fantasmic! at Disneyland – Everything You Need to Know
I do know that you can have someone go to stake out space, but I also know from experience with other parades & shows at Disneyland that by 45-30 minutes before showtime you’ll want your group with you to make sure that they have enough room & don’t get crowded in by groups who show up all together.
As for after the show, it is a pretty gnarly crush getting & out. We usually stay & enjoy fireworks that are right after, but unfortunately now thru March fireworks are a no-go while they repave Main St & replace the track for the trolley. The west side of the park has Haunted Mansion & Pirates of the Caribbean which are both people eaters that you can escape to after the show while you wait for the crush to resolve. Pirates being a 15 minute boat ride will buy you some time for the crush to sort itself out but if it’s still not quite sorted it’s right in the middle of it afterwards so a better plan would be to head towards Haunted Mansion.
Critter Country will be nice & quiet too, but with Splash being down for refurb only Winnie the Pooh will be up. It’s a great dark ride though & almost always a short wait so if you like Winnie the Pooh, this is a great time to enjoy it. If Splash does happen to be back when you go (it’s usually down for a refurb for quite some time this time of year), that’s also a great time to do a Splash marathon as it’s usually near a walk-on that late.
No rides close during Fantasmic for Fantasmic. If the railroad is closed it would be related to fireworks and the wind direction, but you shouldn’t have to worry about that since no fireworks are scheduled while the work is being done on Main Street. Doing a loop is a fantastic idea too to get out of the area without having to deal with the incoming crush for the next show.
During the show, Pirates and Haunted Mansion and most of the west side of the park are low waits. However, you will want to watch out for the potential to get caught in the crush in between the two shows.