Hi -
Have you thought about having your options be 1 and 0? Then you could sum all the fields and if the total = 9 (9 fields, right?) your text box could be Yes. If the total is less than 9, your text box could be No. (As an aside, if you don't need to store that value for anything, perhaps use a couple of expression boxes instead and conditionally format them?)
That being said, what you suggested should work. I'd guess your issue might be with case (if your option has a value of 'yes' and your condition is 'Yes', that would evaluate to false). You may want to verify the values for each button are what you think, case included, and try again....
Oh, and the 5 conditions thing? You can work around that. Change the first dropdown in your condition to 'The expression' then type away, like this:
my:field1 = 'yes' and my:field2 = 'yes' and my:field3 = 'yes' ....
That's three conditions right there, but only using up one line in your 'allowed' 5 conditions...