Excel Like Navigation In HTML Tables - Override/intercept Combo Box Keyboard Inputs

An existing JSP is generating the HTML page and table. I have retro-fitted it so that it adds to each input field a 'trigger' to the onkeyup event.

The purpose of this trigger is check for the the cursor keys being pressed (UP, DOWN, LEFT, RIGHT) and then if it is a logically valid move, shift to the next cell in that direction.

Now, the function it calls, doOnKeyUp, for the most part is working fine... when the input field in the table cells are a textbox or textarea, my method can re-focus the fields appropriately.

HOWEVER, I am having a huge headache when the input field is a combo box. Basically, the value of the combo box is changed with the cursor keys (which is default combo box behaviour (I think)), instead of moving to the next editable cell in that chosen direction.

Problem #1: My question: Is there a way to intercept/override the various onKey events or similar for the combo boxes. For the combo boxes, I want to continue to move between table cells UNLESS a do something specific, like press the spacebar to entire an update/modification mode for the current cell. Everything I have done so far does not prevent the combo box events from triggering.

Problem #2: Also, I tried a nasty hack to try and get it to work, by trying to switch the combo box's value back 1 spot (if it appeared to be needed) by changing the <combo box>.selected value. However, that change never appears to be taking hold, so it is staying at whatever the original selected value was. For this one, I am just curious as to the correct way to have javascript change the selected value in a combo box and make it "stick" and make it applied to the view/UI?

Code example:

You can probably tell by the code that I had been trying various things to change the combo box's selection via javascript, but none has worked for me.