The meaning is similar. "Since then" places more emphasis on "then" (the beginning time in the past). "Ever since" places more emphasis on the time that period extending from the past to the present ("ever").

'since' is to 'then' as 'so' is to 'that' "I'm going to wait until I'm older, since then I'll be wiser" "I'm going to go to the store, so that I can buy some milk" You put 'ever' in front of 'since' when the thing has already happened. "I can't bring myself to watch that movie ever since my father died" or "(ever) since my father died, I can't bring myself to watch that movie" In the second case you can omit saying 'ever' because it's obvious

You can, but to me ever since implies a more definate occurence happened. Since then more makes the time when it happened. You can also add the two. Ever since then.

Yes they can, but like as already been stated since THEN is generally used to add emphasise to the time something happened in the past. Also you might consider adding some punctuation when writing SINCE THEN to force "punctuation readers" readers who follow YOUR PUNCTUATION to take a short break a breathe for it to sink in. This may prevent some readers thinking that the WRITTEN THEN is a separate erroneous THEN=NEXT. Because speaking is much faster than reading "SINCE THEN" is automatically understand to mean "since-then" I was once served rotten food at a restaurant since then!! I have never returned. I was once served rotten food at a restaurant since then, I have never returned.