i agree, the vet should have a look at the runny nose - just in case it's the onset of flu or something (not to say it is!)
wood chewing is a perfectly natural behaviour, and all horses at some point in their lives do it, though for domesticated horses most chew wood because their diet is not nutritionally balanced, being deficient in certain nutrients which they then try to gain from the wood. excessive wood chewing deforms, and can damage, teeth, and can increase the acidity levels in the stomach unhealthily.
therefore, weigh what you are feeding and check what the label on each feedbag recommends as to how much to feed your horse - if you are not feeding the recommended levels (which is likely) then either add a vitamin & mineral supplements (such as NAF Pink Powder or Global Herbs Globalvite) or a feed balancer (such as Baileys LO-Cal or Blue Chip Lami-Light or original). these options are both a good idea if you are deliberatly feeding small amounts due to your horse having laminitis in the past (to to help prevent it), but feeding more of a high-fibre, low-energy, starch and sugar forage-based feed such as Dengie Hi-Fi Lite or Alfa-Lite, will be mpre beneficial amd healthier for your horse (visit the Dengie website via google for more info)
- or you can increase the feed until she is receiving the recommended amounts.
you may well find a positive change when an antacid is added to the daily feed (such as Feedmark Settlex), especially for horses who are not being given enough forage - they often chew wood in an attempt to try and gain more fibre. feeding forage ad-lib or at least plenty of it (in large, small-holed haynets constantly availiable if her forage needs to be restricted).
it is true that true crib biters cannot be cured - a lot like smoking; the odd fag goes a long way to relieve stress, and though it can again cause an increase in acid in the stimach in a viscious circle, it should not be discouraged by shouting/clapping, etc, as this will only increase her psycological need to do it, but every effort can be made to ensure they are as comfortable as possible to reduce the chance of them crib biting to the maximum. you've probably found that the wood chewing started/increased when your horse was brought i 3 weeks ago - why not keep her out? she is (i presume) out in the day, what makes the night any different? in fact, keeping ehr out allows her to adapt to the temperature changes more easily
there are plenty of toys - both homemade or manufactored - to keep her occupied and as stress-free as possible.
to name just a few:
slices of fruit and veg hidden in haynets
baby bottles of peppermint solution wedged inbetween bars
a branch of birch/willow/chestnut for ehr to strip and chew as she pleases, as done in the wild
apples bobbed in water buckets
constant companions, at least who she can see - if they can interact, all the better
(lots of turnout with companions, preferably 24/7 - you may find that this even 'cures' the wood chewing by reducing her stress levels so much)
several whole fruit and veg hanging by baler twine from the ceiling at different lengths
these all will stimulate her brain and ability of choice and keep her busy for hours
good luck!