spontaneously there's only one way I can think of to do this:
First you'll have to set up a URL-parameter. This changes your survey link to http://linktoyour.survey/uc/survey/?a=
Be sure to include this when sending out the invitation links at the first time.
At the point of the survey where you want to, set up a textfield where the participant puts in his/her email-adress.
On the same page, you set up a mailtrigger that sends a mail to the participants with the invitation link to the next measurement point at the desired time.
Here you include the survey link in the following manner:
This will send out the invitation link to the next measurement point and include the participant's consecutive number, so you can match the different measurement point's data.
Now there's one last step to be taken to comply with your data protection issues.
On the page following the one with the mailtrigger, you set up another recoding trigger which recodes v_x with an empty value (where v_x is the text-field variable which was used for the email-adress before). Set this trigger to the execution position "directly".
What you get from this setup is:
1. a way to match the different datasets belonging to one participant
2. the email-adress will be put in, but when the participant submits the page, which would generally lead to it beingt stored in the database, the recoding trigger on the following page will instantly overwrite the information