I created a form with 288 picture boxes (at runtime) on a single form with no visible slow-down. (each picture box contained an image loaded from disk)
My thought was to do all the DB updates from a web service, but you can still do it all from a web form.
What I don't get is why you want a Windows form to call a Web form in order to update the DB.
I wouldn't bind the textboxes for something like this, get the PK id from the selected row in the DG, then use the related dataview or datatable to get the detail values into them. (the textboxes)
Add the Placeholder to the page then add the DD to the PH then set the PH visible to false if the checkbox is checked.
For the checkbox, simply add an event handler and set the autoPostback to true.