The only marinade put on the ciabatta is the one tablespoon reserved, correct? Is that really enough?

Along with a healthy drizzle of olive oil to soak into the crumb, it ends up being quite delicious once grilled. I wouldn't dissuade you if you wanted to make a small bit extra marinade initially, but my approach was: slather the shrimp with the marinade, dot+smear bread with the remainder. And since they're eaten together, there's some marrying going on too. Let me know how it works out!