Not sure what you mean by “main”, but I sure don’t think SS considers La Terrazza the main dining venue. For example, breakfast can be taken in Atlantide (full service), Terrazza (buffet), Arts Cafe (akin to grab-n-go), or in your Suite.
Agree with Mysty. Familiarize yourself with SS’s website. All of restaurant descriptions are there.
Impossible to answer. Many here think they are both worth the fee, many do not. Some think La Dame is worth it, Seishen is not, and vice-versa. So, check them out yourself and report back.
This forum’s search function is your friend. Loads of menus have been posted here in the past. And, they are all loaded on a super popular website that serves a similar purpose to CruiseCritic, but cannot be named here that we refer to as Voldemort. Hopefully you can figure out the cryptic reference. If not, use the search feature and read the posts and it should become clear.