Hi Lisa, from the information you have provided, I would have advised you to stick to a neutral shoe, as your only pronating slightly on one side and not all on the other. If you then run in slight support shoe or full support shoes, it will simply push your foot arch up, and this can result in knee pains quickly. Especially if one does not need support. In the longer term this could result in injury.
If your old shoes cause you knee pains, even though they are neutral, it could be either due to your marathon training or because the sole/cushioning is gone. But this would be hard to tell by me, as I have not see the shoes.
Brooks ST5 of Saucony Fastwitch are the best ones on the market as a Racer with slight support.
A lot of runners use racing shoes not just for their races, but also for their speed training. With a so called normal shoe used to put in the milage training for a marathon for example. The advantages is that you are protecting your body more when using a normal shoe when running a lot. I prefer using racers for the race itself, not especially because it will make me faster, but the lighter weight acts for me like a placebo effect.
The Newton Momentus is a great shoe, but I simply don't think they are good enough for pure trail running up north. I use mine as a winter road running shoe with a bit of park running. For this they are perfect.
And the comment for Boulder trail running design, that is spot on. If it is a flat trail run with no mud, that they are pretty good, but have it wet, and proper english hills, then I would not choose to run in them
sorry to hear that you broke your anke, but from the sound of it your on the way to running again.
To answer some of your questions:
1) Depending on the age and usuage of your old trainers, this could result in you suffering from blisters and knee pains. Knee pains are generally the first sign that trainers are worn down or simply the wrong shoe for you
2) The GelKayano 18 is not extreme over pronators, it is for mid to full pronators.
3) If your gait was analysed as a neutral runner with only one side pronating only just slightly, than a neutral shoe would be the best option. I would not use any shoes that gives you any support.
4) I would recommend a neutral shoe such as a nimbus for example, that is if you want to stick with Asics for example. There are of course plenty of other neutral shoes available on the market all different in fit and distance build for.
On another note, I highly recommend for any neutral runner to stay away from any shoes that are designed for runners who pronate, simply because if you don't need any arch support you should not have any. To much arch support will result in the long term in knee problems and shin splints. Your comment that wearing structured shoes coud mess with knees/hips and ankles is absolutley correct.