Yeh, I take your points. The rumour actually came from someone I know in Spain who had heard mutterings from somewhere. It is more a case of IB being interested in some kind of agreement rather than the other way around. Don't really know a great deal about Iberia or codeshares so not in any position to comment further.
With regards to the reason why ZB and IB work so closely in Spain, I do think it is a little more than the fact that IB were once the only ground handling option. There have been much cheaper alternatives for a good number of years now, particularly at the larger airports, so Monarch are obviously very happy with what IB provide and the connections to such a large company. I remember seeing a poster at TFS a few weeks back which said 'Monarch - proud to be handled by Spain's National Carrier' or something to that extent.