It's pretty simple really. Had it not been for Bale starting to break through, vdv probably would have been our best player that season. Even though he was out of favour at Madrid, he was still a talented enough player to go anywhere at that cheap price.
As for eriksen you're right. Everyone knew he was going to leave ajax. But for years he has been a top rated prospect with a lot of big clubs interested in signing him. Even city made a solid offer and would have paid him probably double what we are, yet we signed him.
I don't see Martinez or Alexandre Lacazette being out of our range at all. The French league is abysmal and there is a good chance we could get a cl spot this year, so I doubt Lacazette would be too big a problem.
Martinez has just moved but had we moved for him in the summer, we may have landed our man. Chances are he wasn't the type of player Poch wanted though.