Why is this structured as 'finns det?' Is it the same to switch it around as det finns?
Not unless you put that first: "Det finns alltid mycket mat på smörgåsbord"
Man kan säga "det är alltid mycket mat på smörgåsbord"! :
On doesn't sound quite correct to me. Perhaps better: At a smörgåsbord...
Is it wrong to say it in plural smörgåsborder ??
Yes - the plural of bord is still bord, and the same goes for all compound nouns using it.
Doesn't finns det make this a question rather than a statement?
No, på smörgåsbord is the first unit of the phrase, so the verb moves to second position instead.