You may also find this Microsoft Community Office forum post to be helpful. There is substantial discussion about the trick of using text boxes over combo boxes to do what are often called Cascading Combo Boxes, and by persons who I am acquainted with and have the utmost respect for their technical skills. The general consensus is that making them work, and work well, especially at the complexity level you have is a daunting task.

I also have not had any positive experiences with the split form feature that was introduced in 2010. I much prefer to use a subform (or subforms) to display information about a selected record based on the selection of a single record in the main form. To select the main form record I generally use a treeview, or an unbound combo box to filter on a specific field and then display a single record or multiple records depending on the design. Let us know if you do finally nail down a solution.