Simpson Bay is tough, as far as best prices. The large store in Simpson Bay is Market Garden, which in general is a ripoff. Best prices for groceries on the island is Super U in Marigot. Best big store on the Dutch side is Carrefour by Mark's, close to Phillipsburg. The smaller Carrefour markets, especially the one at Jordan Village in Cupecoy, have decent selections and decent prices.

As far as premade sandwiches, they have them a bunch of places, including Market Garden, Maho Market, etc. Or buy baguettes and lunchmeat, etc., and make your own.

Seaweed varies.

I'll let someone else try to address some of your other questions.