There is a basic problem with your recommendation. If your software is designed to accept "independent loadable components", then each component could have a legally bounded identity that can be separated as not being part of your software (also you need to choose the right type of license).
I am not totally sure what you mean by this, but I think you mean someone could submit or utilize a reusable module from another product. If so I don't see how that effects legality any more than a contribution to a web site. Someone could post an illegal poem (e.g inciting racial hatred) that could also be part of an anthology, maybe legally published in a different country. The site owner still has to remove it when notified.
If the contribution becomes part of your software "genes", then it is extremely difficult to proof that the resulting product is not infringing any type of legal definition. How can you say the line 300 and 450 is not our responsibility?
I think this is a genuine issue, but not in the way you describe. If there is a single line or module (like decrypt RTMPE in the example) this can obviously be removed, and other streams left in tact. There could be a project where the whole purpose of the software becomes the illegal activity - it is listed as an RTMPE downlaod utility, other options become unsupported etc. If this happens then it does become difficult to say "we are not responsible". I see this as the same as when a web-site becomes a hate site or pirate site. In most cases it will be clear cut - but there are bound to be some difficult ones
Obviously this is a combined problem : "design" and "code management". Maybe when we define these types of things on new projects, we need to think in possible future legal issues and work according with that reality.
I would hate to have to have things designed so that they couldn't be used illegally. we don't go to that extreme outside software though so it shouldn't really apply here. Why do I not trust the legislators though?