Just to jump in with a few technical details:
Okay, for starters, "DRM" is a heavily overused term to the point that it's nearly meaningless. It is accurate to say that it does give finer-grained control over what can and can't be done with the content, but we should distinguish between "the current implementation" and "DRM as a concept" because it's fairly straightforward for us to change minor details and (as I've indicated in the past) we are looking into the various concerns and taking appropriate steps.
The rest of my answer here refers to the implementation in your hands currently (in TS12 SP1), not to DRM as a concept, and not necessarily to future implementations.
1) Changing the whistle volume, hornsound ...etc ... on the NKP Berkshire.
You cannot modify the original payware files directly, but you can make an aliased version that depends on the payware files for the model, textures, etc. but substitutes in your own horn or whatever. This means the simple approach of "open the original file and modify it" won't work, but we discourage that approach even without involving payware. In the future I'd like to see this made simpler by allowing in-surveyor reconfiguration of things like sounds and engine specs.
2) Weathering, Reskinning, and correcting the misspeeling of the "Nickel" Plate Road caboose.
You would have to make an aliased version of the asset and use texture replacement. That's how you should normally approach reskinning anyway, so status quo.
As far as using the original texture files as a stencil for painting over, that opens up a whole can of worms. It's never been possible with any JA-packaged Trainz content, although there are third-party tools which have made this possible. It's also unlikely that this is legal unless explicitly blessed by the content's creator. Our current payware packaging system also does not provide any explicit support for this kind of thing, though there's certainly nothing stopping the original creators from distributing the necessary stencils if they choose to do so, and there's nothing stopping you from reskinning without the stencils.
As far as redistributing the payware mesh files, that's never been acceptable.
3) Locks you out of altering all files on DLC assets.
Correct.
4) Does it lock you out of reskinning, and reworking ALL of Trainz: bell/engine/horn sounds ?
This appears to be a rewording of the above. You cannot modify the original but you can make your own variant which uses the original mesh/texture but a different horn sound or other details.
5) If, and when, N3V .. should N3V ever go bankrupt, all your DLC assets will stop working, and the game will stop working all together, because you can not connect to their server ?
TBQH, if we all dropped dead today and nobody took over the IP, then yes, the payware assets would stop working after a month or so. Multiplayer, chat, the DLS and forums would also fail within a few weeks. The game itself would continue working until you upgraded to an incompatible computer/OS. To be fair, though- that event is exceedingly unlikely and there are ways to mitigate even that problem, though we haven't taken those steps as yet.
kind regards,
chris