My problem will be when I get to the really complicated pdf's with radio buttons and calculations.
I would probably try to recreate the whole form in Excel rather than using the original as a background image (unless I've misunderstood). You can use the design toolbox to add radio buttons, and formulas for the calculations.
For the straightforward data entry fields, you can create a master sheet with a field for each necessary entry. In the sub-forms (all the rest of the sheets) you would just refer to the cell on the master sheet by including the sheet name with the reference. Excel will add in the sheet name for you if (for instance) you type an "=" in the cell on the sub-form, then click the "master sheet" tab, select the cell with the information you want transferred, then press Enter. You probably already knew that...but just in case
BTW, how is a doc easier?
My assumption is that you could simply copy a section of a word document as-is, then re-format the information in Excel as needed. I'm not sure why Excel won't let you change the contents of merged cells...I try to avoid merged cells as much as possible, but sometimes it makes sense to use them. I've never had a problem not being able to change the contents, though.
Regarding the amount of narrative...how much information needs to go into a single cell, and related to that, what version of Excel are you using? Newer versions of Excel (2007 and 2010) can handle much more information in a single cell. Is the problem that the cell needs to re-size to handle varying amounts of information that might be entered?
I don't feel like I'm being much help, but I'll do what I can to help you find the best way to accomplish your goal.