Sounds like what you want to do is bulk edit of the last column for one or more rows (if they're checked). Is that correct?
If so, you might be able to use a nested eval(eval(...)) and xpath to iterate through the list and set them but it's possible you can't easily do this with out-of-the-box InfoPath.
We created qRules to address common functions that people need that require code. After injecting qRules into your form, you can just use rules to do this kind of iteration. I believe Jo-Anne Alfafara has a nice blog on the Insert command and how to copy items from a SharePoint list into your form. The same technique could work for you. Get the count of the rows in your list (use xpath count() function) and set that into a field (you can add a field to your main data source *or* a secondary data source which has the added benefitted of keeping your main data source pristine). As you iterate through the rows you have a condition that sets the value if the checkbox is true. Rule based iteration is a little tricky but it can be done. The other qRules command that might be easier is "SetValue"; this was developed for exactly the purpose you intended. To set all values that match a filter. I should have recommended that first. That's a piece of cake. Install qRules free trial and look at the user guide.
Note: qRules does not work with InfoPath 2003.