see this tutorial for details on using multiple update:
i'm not sure what you mean about the checkbox, but if a form element is not inside of the form, it is not a part of the form submit.
are you trying to use the checkbook to determine which records get updated? it doesn't work that way, all records. an update page does not update based on what is changed, an update page should pre populate using the original data, then when it submits it adds that data back into the recordset, if you made any changes, it will include those when adding the information back to the database