One of your executed commas took "the executions" along with it; tarred with too broad an eraser. That aside, I think it's a good argument for the Strunken Whitean comma before the conjunction in a list of three or more items. I'd go with
Quote:
The shock tactics, the arrests and repression, the executions, and the defeat of invasion attempts all combined to overawe any possible opposition and stunned the population into submission.

I don't know if the last two commas

"... attempts, all ..."

and

"... opposition, and stunned ..."

are good from a formal point of view, but I could see where they might lead to confusion in the mind of a non-native speaker, even one as fluent as you are, BranShea.