Ok you're right, I'll take another stab at it.
Do you think for a moment that anyone with more hours than you who needs a job is going to pass on whatever 135 gig you hope to get 'because some other pilot might need that stepping stone job too'? For that matter, do you think that anyone with more hours than you who needs a job is going to pass on that part 91 gig if that's all they can get?
You keep saying you don't want to hurt the industry. But that's kind of like a mobster getting on the witness stand and saying that he didn't want to kill all those people. If you don't want to hurt the industry, then don't hurt the industry. Working for free hurts the industry. You don't have to be an algebra teacher to figure out the math in this situation.
So in short, yes I think its better to commit to a job and then leave when a better opportunity comes along than it is to prostitute yourself out for free. You have to take care of yourself. But you also have do whatever you can to take care of the industry you hope to someday eek a living from. Don't work for free.
Let's face it, even if you posted your offer for safety pilot/free flight instruction everywhere on the net, it would still probably take you a good six months at best to accumulate 50 hours of night time. If you get a job at a busy flight school and schedule as much time at night as possible, you could probably pick up 50 hours in less than 3 months. And if its a busy flight school, they're going to so used to CFI's rotating out after a couple of months that they won't even notice when you're gone. And that's only one possiblity. There are others. Find a 135 that uses F/Os and pump gas on their ramp until an F/O quits. F/O's don't need to be PIC qualified and once you're in the seat, you'll be getting twin turbine time instead of humping frieght around all night in a tired cherokee six. The point is there are other options besides giving it away for free.