In general, I would advice performing Fat grafting in the lower eyelids and Rhinoplasty at separate procedures. Thre would be greater predictability in results of the lower eyelids.
Predictability of fat grafts in the lower eyelids is subject to many factors, some of which are unknown. If the lower eyelid depressions are minimal and if there is herniated lower eyelid fat present producing fat prominences, it is possible to use this fat to make fat flaps.
In other words, I would dissect the herniated fat and bring it forward into the depressed areas of the lower eyelids. In my experience, this is preferable to actual fat grafting into the lower eyelids. I have performed lower eyelid Blepharoplasty with fat flap advancements in conjunction with rhinoplasty in many cases with excellent results.
The loss of 20 lbs. after surgery depends on your basic weight. If you are extremely overweight, loss of 20 lbs. would most likely result in no adverse effect.
On the other hand, if you are very thin and underweight, 20 lbs. weight loss is a relatively large amount to lose and might effect a change. By the nature of your question, I assume that you are overweight. The loss of 20 lbs. may not present a problem for you.