You should be able to check in early in the morning already. The counters may be closed in between flights (there is one at 8 and the next one at 12:15, so guess the counter might be closed from 7:30 until 9:15 or so). Try giving them a call to check. http://www.schiphol.nl/Travellers/Fl...hol/Iberia.htm

If early bag drop doesn't work, there are bagage lockers in all sizes available at the airport (http://www.schiphol.nl/Travellers/At...ageLockers.htm).

Your IB flight will depart from the B (Schengen) pier, which has a common security check. There may be some queuing around 18:00. Just allow for a bit of extra time to be on the safe side.