You're asking for opinions, awesome, I have lots of them and have a special love - hate relationship with Dr. Luke!
But here's the warning, I was lucky enough to have a very healthy and easy twin pregnancy and birth. Not that I expected one, but in hindsight, it colors some of my opinions and makes me maybe a little more cavalier than I should be. Maybe I look at my twin pregnancy as especially easy since I planned for a difficult one, having read the kind of things you ask about.
Dr. Luke and her book: well. Yes, I think she's onto something with her push for weight gain and nutrition. In my amateur opinion, don't worry about weight gain in the first trimester. But as soon as morning sickness subsides, if any, make sure to eat well. Drink more fluids than usual, and plenty of protein. Be sure to give your body what it needs. For me, I carried a bag of nuts around and had a protein shake as a snack at work. Lots of good veggies. I wasn't diabetic, but tended to that kind of diet - less processed carbs, more whole foods, minimize sugar. I didn't count anything, but tried to eat plenty and well. The weight really piled on in the second trimester, but seemed to plateau for me at around 30 weeks. Maybe that was perception, once you get that big it's all relative. I lost 2lbs in the first trimester, and ended up gaining about 24# by 24 weeks. Total gain of around 50ish lbs. Can't remember exactly.
Activity level - I planned on putting myself on modified rest at 30 weeks. No social activities and no traveling. Just work and rest. Announced my plans to friends and family. But I found I didn't really need to. I was feeling good, and walked 2-3 miles a day right up to delivery. Those walks, with great shoes, were really great for me. They kept me healthy despite my size. Stayed working full time at a low stress office job until 2 days before delivery at 39 weeks. Didn't really nap.
Is there a relation between activity level and preterm delivery? I'm not convinced. There maybe a link between maternal stress and preterm delivery, maybe. Are you on your feet all day? Is your job stressful? Then you might want to plan on scaling back. But if you don't have leave time (like me) and your body isn't giving you any signs, and your cervix is long and strong, then keep doing what you feel you can.
IMO, preterm labor happens. Why? There are a lot of theories, and each may have some truth to them. And they are all strengthened by hindsight. The mom in Dr. Luke's book who worked all those long days and that's what brought her babies early. Who can say that's right? Problem is, there's not enough science. What is KNOWN is that bed rest doesn't prevent preterm labor. There's a time and place for bed rest. But in general, it's not helpful for otherwise healthy pregnancies. Could be harmful.
My recommendation is to expect a healthy pregnancy but give yourself some wiggle room. Set your boss up for the possibility that you will need to take a less active role. Give yourself plenty of outs, but listen to your body. If you feel good, keep doing what you're doing.
And if your babes do come early, don't blame yourself! These things are incredibly complicated. Know that you've given your twins the best start you could.
Finally in this novel of a response, I'll shake my fist at Dr. Luke. Grrrrrrrrr! I believe she's responsible for the study suggesting mortality rates for twins goes up at 38 weeks. This is sloppy science, and harmful advice! It may be true that taken as a group twin pregnancy complications increase after 38 weeks. But that includes MoMs with known pre-existing complications like GD, preclamsia, etc. In otherwise uncomplicated di-amniotic pregnancies, that number is closer to 39 or 40 weeks.
To sum things up, here are my two favorite links:
A study about twin gestation and delivery-http://www.uptodate.com/home/content...=labordel/5122
A blog post about going full term -http://www.lookydaddy.com/weblog/200..._that_mos.html
Sounds like you have a great practice! Stick with them. Congrats on your twins!