Could you explain when you would use en + le gérondif as opposed to tout en + le gérondif? Thanks!

I'm still not convinced... Is there really no difference?

From the examples, I notice the ones with gerondif on its own are not just things that happen simultaneously; e.g. you could not simply put the gerondif "on the other side", except in the "going to the market" one (though the meaning or the emphasis could change a little).

1. En allant au cinéma, j'ai croisé Mathilde.

2. Il est allé au marché en courant.

3. Je me suis blessé en me rasant.

4. En faisant mon lit, j'ai retrouvé mes chaussettes.

2 -> Here both things seem to occur at the same time all along. Moreover, the gerondif is used here to say how something was done, not really to stress that "going to the market" and "running" where simultaneous.

1, 3 and 4 -> While doing something, something else happened. There's no sustained simultaneity. One thing happened while the other was (and probably kept on) happening. 

So 2 is completely different from the other 3, which can only be ignored if we focus on the English translation...

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5. Tout en préparant le repas de ses belles sœurs, Cendrillon rêvait au Prince Charmant.

6. J'ai eu mon Master tout en travaillant dans un restaurant tous les weekends.

7. Il est possible de faire des bénéfices tout en protégeant l'environnement.

8. Charlotte a rangé son appartement tout en discutant avec sa mère au téléphone.

#5 doesn't seem to sound right without "tout en". Here we're not saying something just happened while another thing was going on, but there's an intention: Cendrillon was doing something else, but went and woke up Prince Charmant anyway. Example #8 is the same case.

#6 uses "tout en" to stress simultaneity between two very different actions: different places, difference type of action. There's not even real simultaneity here, since he probably didn't study at the workplace. This is a much more abstract notion than just saying "While this was happening, that happened".

#7 would translate "while protecting the environment at the same time", and the stress is not on temporal simultaneity, but on how both things are not necessarily incompatible.

I really don't think #7 would make sense without "tout en", and I have my doubts about #5 and #8.

As for #6, without "tout en" it seems to mean "He got his master by working all weekends", i.e. as if working was a cause and getting the Master was the consequence. Similar to "en courant", it'd seem to describe HOW something was done or achieved, and not the simultaneity.

I apologize for the long message, but I really think this lesson needs a little more oven...