I'm no expert and speak with absolutely no authority but I'm currently using a treadmill for C25K for convenience/accessibility reasons, although I've only completed Week 1 and 2 thus far. If I had easy access to level-ish ground I'd give the great outdoors a try. I understand that the treadmill experience is different but that doesn't make it any less valid.

I keep the deck at a 1% - 2% incline at all times and my speed is about 4.8 - 5.0 depending on the day.

At this point, after following the Treadmill version of the C25K, I'm able to do the entire 5K.

Again, I know it's not the same as outdoors with wind resistance and continual grading changes, but it works for me.

Sounds like absolute purgatory to me, I can cope with about 5 minutes on one before I'm bored, but my preference is firmly for trail running, so lots of rough terrain, trees, wildlife and weather

In pratice putting the elevation on the macine doesn't make a hge difference, so I'd really just say that you need to set your expectations about how much a dreadmill equates to real world running. If you're thinking about entering a 5K race, then anticipate that equating to about 8km on the machine, and plan your pace accordingly.

As far as cross training is concerned, rather than an elliptial I'd suggest some resistance training. the effects of that are complementary to the running, reducing your injury risk, and improving your running power, hence pace. My preference is for bodyweight training, using You Are Your Own Gym, although alternatives exist.

It definitely counts, but it's also definitely different than running outdoors.

Most weeks I do 2 days on the treadmill and one day outside. I used the treadmill exclusively for the first 3 weeks and when I tried outside on the 4th week I thought I was going to die The big difference for me was the consistency of pace. It turns out I was running much faster outside than on the treadmill because I couldn't tell how fast I was going. After 9 weeks (I've repeated a few weeks ) I think I've figured the pace thing out, but I'm still going to stick with my 2 days of treadmill out of necessity (time/location/heat).

I did my C25K the first few months of 2014. Began inside on the treadmill and moved outside the last few weeks of the program when the weather was warmer and road/paths less icy/snowy. Ran with the treadmill on an incline of 1.5%. I think if anything, if you're going to run a race, you should try at least some running outside to get used to it.