sum of two variables: whats the formula

Perhaps I'm not reading your question correctly, but it sounds to me like you have the solution right there. Maybe there is an errant Action or something that is throwing you off. If you are able to trim the title down to just the relevant pages (and strip out anything confidential), I'd love to see an upload of this.

From what you describe, it sounds like you could still do the addition at the end. As you have pointed out, you can Set the variable based on the last item clicked. I would definitely assign memo1 to the first question. Then assign memo2 to the second question and so forth. No addition whatsoever at this point.

On your last page, I would set an Action that sets memo_end equal to 0. Reset it so that nothing corrupts your data. Then, run a series of Actions. First, Modify memo_end so you add VAR(memo1). Then Modify it again so you add VAR(memo2). And so forth. For these actions, I would suggest a 0.2 second delay. You probably won't have any problems with the Actions being in order, but I've been bitten before with large, resource-intensive projects, and the math was impacted by that. Make sure that memo_end = 0 first (you could even set it earlier in the project). 

As I said, I might have missed your intent, but I think you have everything you need already. If it's not working, I'd suggest a quick upload so we can take a peek at it.

Kevin