Belgium worth the stopover?

Normally, I would say yes, Belgium is of course worth seeing. Considering your short amount of time though, I would not plan on it.

I would use the extra days in Berlin. Perhaps do some day trips to Potsdam, Sachsenhausen Concentration Camp or to Dresden. Berlin itself is a large city and spending 3 days there is a minimum.

Although I love Brussels, it doesn't really fit logically into the flow of your trip, which is busy enough.

Ditto. I love the food and beer in Belgium, but that's a lot of traveling. You can also consider stopping someplace in between Berlin and Paris, like the Rhine region. When I travel, since I live in a big city (and so do you), I don't like spending my whole trip in big cities, I like to spend at least half of the time out in the smaller towns. Stopping at the Rhine would give you a chance to do that.

I am using easyjet to go between cities now that skyeurope has folded. I thought it may be easy to fly to brussels from berlin rather than going directly back to paris. I'm not entirely sure how many days to spend in barcelona, but i thought-
1 Day in brussels, then the next day I could take train to antwerp or bruges? Then take 2 hr train to paris next day.

Brussels really takes 2-3 days to appreciate, otherwise you'll think all it has of interest is the Grand Place and Mannekin Pis. Brugge you can fit in one full day (but not as a daytrip). Depending on what you want to see, Antwerp can fill a daytrip or several days.