Hi Malcolm, ive taken a look at the pico example i think you have been looking at, and at best it is misleading. UKAT3096's post already explained that a post cat o2 sensor signal voltage will be high with a good cat this is correct. Pull another car in and scope pre and post sensors again,you will need to rev the engine for a surprising amount of time for both sensors to get hot enough to output, and for the cat to flash and start operating, post sensor will go high and level because there will be very little oxygen left after the cat has reacted.
Fuel trim errors can be tricky to nail, because the root problem can be so many things, air leaks, as you have already mentioned, dont forget the crank breather system though, its part of the induction system, slight missfires(for numerous reasons), injector performance, fuel pressure and flow, do not overlook these physical things before you get lost in the 'lambda loop' hth keep scoping and posting.
Flat out air mass figure from live data might be helpful, and i have also seen interference on AMM signals like yours with no running problems.
Regards Paul