Edema refers to swelling in general. Pitting edema is the when pressure on the swollen area leaves dents that are slow to refill. Generally I think of pitting edema as related to older, more stagnant, or non-acute swelling, versus the type of swelling one has in an acute injury, where it is related to blood flow into the area. Pitting edema is going to be more related to poor lymphatic return, resulting in increased fluid pooling in the interstitial tissues.
So, no, pitting edema isn't "worse" it's just a little different.
As far as pre-e, your midwives should be looking at urine dips which will give them information about protein in the urine, which is a more definitive symptom. Increases in blood pressure are relative to your normal BP, so if you usually run on the low side (which it sounds like you do) you may have "high normal" readings that could be indicative of HIGH for YOU. That's what matters. A drug store BP reading is going to be pretty unreliable. There are a lot of little things that can affect the machine readings.
Finally, yes, you can certainly swell significantly without it being pre-e. The increased pressure in the pelvic floor can hinder venous/lymphatic return, and result in swelling in the lower body, which is pretty common in pregnancy. Elevating the feet and wiggling your toes/feet/lower legs will help pump the the fluids back out, as well as having someone gently milk the fluid upward toward the hips. Lymphatic return is a passive system and requires muscle pressure and gravity to get it out, while it gets IN with the pressure of the active blood circulation.
Most large weight gain in a short period of time is going to be due to fluid retention and not fat gain. You'd have to be eating THOUSANDS of extra calories EACH DAY to result in a 10 pound gain that was due to fat/caloric imbalance gain and not fluid.
Hope this helps. If you are concerned, continue to mention it to your midwives and ask them to explain why they are not concerned. They should have a good explanation, which should in turn relieve your concerns.
Co-sleeping, Breastfeeding, EC'ing, Baby-wearing, Homebirthing mama to two fabulous girls 6/2007 and 8/2010