I interpret "opposition without alternatives" as "People oppose my position, but they don't offer any alternative positions." In other word, people are complaining without being helpful.
I agree with your second paraphrase.
The third set of phrases confuses me too. Kennedy (this is the speech he would have given on the day he was killed) was criticizing those who opposed him. His speech later talks about a great increase in armament that his administration was creating against the communist threat. So the first two phrases, "words will suffice without weapons, vituperation is as good as victory" imply that Kennedy's opponents are advocating disarmament, and that talk and diplomacy are enough. The last phrase, "peace is a sign of weakness" isn't consistent with that, as I interpret the phrase to mean that diplomacy is not adequate, and aggression is necessary.
On the other hand, this is a politician's speech, and he is trying to make his opponents sound illogical. Perhaps he intentionally made it seem like his opponents held self-contradictory ideas to make them appear foolish.