Well, I found at least a solution to get me halfway there:
(1) I installed the dynamic text plugin (http://sevenspark.com/goods/contact-form-7-dynamic-text-extension). It is able to populate a form with values handed over by URL, like http://mysite.net/contact/?name=Mikefirstname.lastname@example.org
(2) I modified my form on the entry site (where users fill out the form):
[dynamictext your-name "CF7_GET key='name'"]
[dynamictext your-email "CF7_GET key='email'"]
(3) I modified the form which is displayed on the "check your entries before sending" page. The display of the entries as such remain as before...
...but I added a "back/correction" link:
[form your-name]&email=[form your-email]">Back
This link adds the previously added values to the back link. The "back" link is basically no real back link, but only a newly opened page with the form, but already populated with the previously entered user's values.
(4) BUT there are some downfalls:
- The dynamic text plugin only works for hidden fields and text fields, NOT for text areas, dropdowns... That is, for the classic contact form the possibility of going back to the message (normally entered and displayed in a text box rather than in a text filed) will be really missing.
--> A possible workaround for at least the text field problem could be to have the content of the text field handed over in a proxy/auxiliary hidden value, with this value re-assigned to the target text box. I think this could be done with a short code that redefines or swaps the variables. (I'm not so much into PHP coding, so maybe anybody else knows what and how to do.)
- The URL will get quite long, the more complex the form gets.
--> A possible workaround could be switching POST/GET, but again I'm not very knowledgeable with this.
To cut a long story short: By using the dynamic text plugin, "going back" is possible, but only for text fields. For text areas etc. some more work and workarounds are needed.