I've got a 2007 Stumpy with the brain and 140mm Talas up front.
To answer your questions....
Does it stay locked out too much?
If I wind the dial most of the way round to 'firm' the suspension is very hard although there is still enough travel to comfortably handle chatter associated with uphill or flat ground.
Does it give a softer ride over rocks - downhill and uphill?
I tend to switch settings at the top of a ride if I remember. Generally I ride with mine about 5 or 6 clicks from the full settings uphill and, if I remember, wind it round to full soft downhill. It feels supple enough to handle the hits downhill whilst not being too soft uphill. I have a limited frame of reference though and can't compare to the more modern Pro-Pedal/CTD systems which are out there.
There's relatively little pedal bob when sat down but it's a bit bouncy if you stand up and mash the pedals. I weigh just over 11st so your wife should have a similar experience unless, well... uhm... you get the picture.
Is the Brain adjustable enough?
It seems to be, although I think you'll find one setting and either stick with it or use the settings within a few clicks of that + full firm.
In terms of trail centre/downhill use, I've not found anything to complain about. I've ridden lots of reds and blacks in Wales and never had cause to complain about the bike. The biggest challenge i've probably given the bike is hammering it round penmachno (through the droppy rock garden bits) and I had nothing to complain about. It also handled the jumpy/swoopy black at Llandegla without any problems.
Climbing is easier with the fork locked down to 100mm but at 140mm the bike still climbs well - you just need to get your weight over the nose of the saddle and drop your elbows to keep the front planted.
I've stuck a shorter stem and wider bars on mine and enjoy the more planted feeling it gives over the standard long stem and narrower bars.
I've also stuck a 2.4 on the front and run a 2.3 on the back although I spent the summer running a 2.0 on the back (in case you're wondering how much rubber you can get between the fork or stays.
Off to Bike Park Wales this weekend so I'll let you know how it feels at a more downhill orientated centre when I get back.