Here is a link to the info on the F!. Dining Package.
http://www.wdwinfo.com/wdwinfo/dining/fantasmic.htm
If you read it carefully, it should answer most of your questions.
Yes, you can use 2 TS credits on the DDP for the F! Dining Package at HBD. You need to be careful when you book this. NO NOT make the mistake of booking a regular ADR at HBD. You need to book the Fantasmic package and then select HBD as the restaurant that you want to eat at.
The other two dining choices on the Fantasmic dining package are the buffet dinner at H&V or lunch or dinner at Mama Melrose. These are 1 TS credit each.
None of the other TS restaurants in DHS offer this package.
I definitely think it's worth it as far as avoiding the regular standby line for the show. The only time I would consider not getting it is if I knew in advance that there would be a second show and that I would prefer to go to that one. Reserved seating is only for the first show if there is more than one show on any given night.