Do you really need to have the 'ett' in this sentence?
No you’re right, the article can sometimes be left out.
Can one say "i restaurangen" for "in the restaurant" and have its meaning stay mostly the same?
You should be able to do that. It's even better Swedish
'Boka', 'reservera', 'beställa' and 'tinga' - would they all work here?
I got "jag reserverar ett bord på restaurangen" wrong
"reservera" should work here, imo, though it's a bit rough. As for the other ones, I think not.
may you fix the Numpad functionality for multiple choice questions?
Can you also say "boka in"? "Jag bokar in ett bord på restaurangen".