Just screenshot them instead of embedding them. If you cared about JS injection why would you embed anything?!
How are you supposed to prove the tweet actually happened if you just post a screenshot? It's trivial to forge a screenshot of a tweet to say whatever you want.
If it still exists, just add a link to it underneath the screenshot. If it was deleted, then there's not much you can do, but that's no different than if you had embedded it.