I'm going to start off with a few links to expert responses that you might find informative.
An expert response explaining what causes proteinuria in preeclampsia: http://www.preeclampsia.org/forum/viewtopic.php?t=622
An expert response about whether increasing proteinuria means worsening disease: http://www.preeclampsia.org/forum/viewtopic.php?t=109
And finally, here is an expert response discussing what testing should be done for a patient with proteinuria of a level similar to that of your wife: http://www.preeclampsia.org/forum/viewtopic.php?t=1845
It sounds like your doctor has things pretty much covered in that department, which is always reassuring!
I'm sure you are aware by now that preeclampsia can present quite differently from one patient to another -and even from one pregnancy to another in the same patient. Some people seem to have the entire list of symptoms, others don't notice a single one. Some have hypertension show prior to proteinuria, for some it is the opposite, and there are even others that have PIH and never do develop proteinuria...or more still that seemingly go straight past it all and dive head-long into Eclampsia or HELLP syndrome. It really is unpredicatable. As much as we'd all like a cut and dry answer to whether we'll develop preeclampsia and when we'll deliver, what symtpoms we'll get and how it will all be treated, there just isn't one. I know that must kill you, but trust me, we definately understand! On to the point of all this: your wife's body will react to her condition in its own way. It may be that her kidneys just aren't as capable of coping with the increased pressures, whether they're crossing the hypertensive line or not. It may be that the hypertension she experienced earlier took a toll on her kidneys and it'll take time, likely outside of pregnancy and with blood pressure levels at her norm, before they'll start to recover. In my own experience (not relating directly to proteinuria, but to how differently each person's body reacts) in the first weeks after my PIH diagnosis in my first pregnancy, my blood pressure jumped a large ammount but only rarely crossed into the hypertensive range. Still, this was more than sufficient to cause my placental function to decrease resulting in Oligohydramnios (low amniotic fluid). No one, not even my doctor who sent me for that first diagnosing ultrasound, would have expected that BP's on average of 120-130/80 would have caused that. My kidneys seem to be able to handle more than what they've been through, yet in that pregnancy, my placenta and thus my baby were the ones being impacted.
I know this is not the answer you are looking for. Maybe someone with more technical knowledge of this area can explain things in more detail. I am not a doctor and definately am not an expert. I know that the uncertainty of it all can just wrack your mind continually with thoughts, worries, and questions. I really do wish that I had the answers...so many lives would be saved, so many families intact, so many better prepared for what lies ahead. I pray that they will come for us all soon.
You and your wife are doing a great job. You are learning all you can and be vigilant about symptoms so that she can get the best medical care possible. While it may never feel like enough, it is so valuable and exactly what needs to be done.
Sending my prayers that the proteinuria will not increase drastically, that her blood pressures will stay down, and that she and baby will both continue on to full term in good health.