Well, the Prune Growth use of Diminish Plants (PHB, Druid 3) could shrink, prune, and untangle a lot of stuff within long range with each casting. That could potentially negate the slowdown or thorny hazard for that area (though I imagine Avril could only cast it a few times per day, so it might only help for a few hours or less each day for all I know). It would make it easier for Warren and Pender to hack and burn through the stuff quickly too, so I imagine we could avoid damage in those areas easily enough.
Summoning thoqqua with SNA 3 could clear a decent path, since anything that touches them takes 2d6 fire damage, which I imagine would burn away a lot of dry bushes and thorns in the Thornwaste pretty quickly. Though each summoning would only last half a minute or so, so that's a far less effective prospect than Diminish Plants.
Air Walk (Druid 4) could get us across a decent area with each casting, but would only affect one subject at a time (casting it on the horses would allow some of us to ride along), but again, that's probably less effective than Diminish Plants.
BTW, what does Cold Comfort do? I don't have Complete Arcane and it's been many months (a year or so?) since I last looked through the Warlock and their maneuvers there (for a tabletop game I was running, where one player was making a Warlock as a backup if his Druid died).