For example, if the PCs were communicating with the dying NPC by magic mirror (and didnt have instant mass teleport), that would be ok? And it would be ok because it sidesteps the possibility of healing?
Or would the magic mirror still be problematic because it's pre-defining what the players "should" feel and takes away meaningful choice?
These sorts of questions indicate to me that you are still primarily seeing my interaction with you in terms of me calling railroading badwrongfun, and that's not really my point at all. It's not an issue of what is problimatic or what is ok. It's an issue of what is skillful, doesn't destroy suspension of disbelief, doesn't tend to cause player drama and what is done with the best of motives - where here I would define best of motives as 'you want to empower the players' or 'you are putting the players desires ahead of your own'.
I'm not really that vested in the whole 'if you aren't running a sandbox you have badwrongfun' or 'if you are running a sandbox you have badwrongfun', and I think I indicate that you can engage in railroading without having a campaign or adventure which can be best summarized as a railroad. Of course, I'm not wholly convinced that a railroad couldn't be fun. However, I am wholly convinced that there have a lot of bad DM's who've used railroads. I'm also convinced that there are far more railroaders out there than believe it of themselves. My pet peeve is DM's who think that they are running a sandbox when they don't prep their games. Drives me nuts. Nothing is more likely to drive me from the table than the guy who thinks he's a great extemporaneous sandbox DM.
As for your specific question, I think that a 'magic mirror' would be one example of a more creative solution to the problem than a fiat 'No'.
A 'magic mirror' like solution is used in Star Trek II - Wrath of Kahn, to allow Spock to have a death scene in which Kirk cannot interfere. In this case the barrier is lethal radiation rather than distance, and the means of communication is an intercom and a transparent barrier, but its serving the same purpose. The secret to getting away with this is plan well ahead by doing a Checkov's Gun trick, so that when the magic mirror finally gets used as a plot device it will seem natural rather than artificial. If you spring these artificial barriers to player action up at the last minute, then they'll be seen for what they are. If you hide your rails, it will all look natural and the players will give you the benefit of the doubt.
I mean really, when it comes down to it, the house rules are just a version of this. From one angle, they are just me telescoping a plot point so that when it happens, it won't come as a suspension of disbelief shattering suprise. Of course, by making them random I'm also allowing the plot to develop in ways I didn't anticipate and can't control.
There are other ways to do it. You can introduce a villain who is particularly sadistic and leaves people behind to die in particularly slow and cruel ways. You can have the players encounter his handy work a couple of times with victims who are already dead, and with strangers. Then you can pull the trick with an NPC that they've developed a relationship with, and now you've got a doubly powerful effect - you've created pathos over the death of an NPC and you've enhanced the stature of your reoccuring villain and made it personal.
But the point is that thinking about these things ahead of time in some fashion is almost always better than just breaking the rules and saying, "No. You can't do that." Maybe you can get away with it sometimes, but that's a bad drug to get adicted to.
Railroading is neither good nor bad for me. It's all how you do it. Vyvyan Basterd is completely right about one thing - DM's must use fiat force all the time and so can't avoid railroading to some degree. He fails however to see where the qualitative differnces lie (and I'm going to have to answer him at some point, but not just at the moment).