Well, before I could make any progress on my two reliability issues (a and b above) one of my Cobalt point motors just stopped working

. It has not been abused in any way and is operated using the recommended 9v DC voltage (checked with a meter to be 9.4V). On top of the other 2 faulty ones (see post above on Dec22) that's 3 out of 7 that have been faulty now!!! Luckily they are guaranteed, so I was able to swap it for a new one without any fuss (though as it was bought online I had to pay postage back to the shop). Anyway, the new one slotted in without any bother is now working fine. But 3 out of 7 is not a good failure record is it? I think I'll have to carry a spare...
As regards point (a) above = unwanted uncouplings, I found this very helpful post on another forum (top post on this page
http://www.rmweb.co.uk/community/index.php?/topic/48728-dapol-easi-shunt-magnetic-couplings-in-n/page-14) as follows:
Benn wrote:Hold the pin with a pair of tweezers and gently rotate it, it should move easily enough to the correct position, and it's sometimes worth a little play about and adjustment sesh to get them to operate that little bit better. I've found a reasonable improvement in operation and reliability can occur by putting a little 'drag' on one axle of each vehicle that has the couplings attached, I've used some spongy laminate flooring underlay and just sandwiched it between the centre of the axle and the top of the bogie moulding, and stuck it on with a bit of bostik. The vehicle now doesn't roll away or move when being coupled/uncoupled, and also remains tensioned to the loco when being hauled so when passing over the magnets it doesn't auto-uncouple when you don't want 'em to!
So I had a session bending the pins outwards to reduce the likelihood of the couplings opening when not wanted, but I still had problems - some wagons were fine, others not. So then I tried adding a small piece of foam under one axle to increase the drag on the vehicle.
IMG_2488 by
PinkNosedPenguin, on Flickr
Not pretty underneath, but invisible when on the track of course. This worked amazingly well

It ensures the couplings stay 'in tension' when running over a magnet and have so far totally eradicated unwanted uncouplings

Intentional uncouplings are not affected and are very reliable. It also has the happy side effect of making coupling-up nicer - you can just gently nudge up to the vehicle and couple up without having to give it a 'big bump' - again the extra 'brake' makes this work better. As I will only run short trains on my layout I don't think I'll have any problems with pulling the rolling stock, even up my future incline. So thanks to Benn (and I think Karhedron suggested this too) for such a simple, cheap and effective solution to this annoying problem. Unwanted uncouplings are so annoying

and embarrassing

its so good to avoid them! Hands-free shunting is a bit of a revelation in N gauge, but is now a reality.