The Problem is:
Ask 10 Dealers and you get 10 different answer. It will depent about how deep each dealer is into the matter. Some Dealers are great in caucasian antique rugs, other in turkman, other perisan tribal, other fine persian. It all depends on experience, there is no book in which you can learn about it. All written is only a glance of knowlege that comes from dealing with rugs for decades every day.
The rug you refer to in your last post is no habibian in my opinion (even if it thought to be). It is also not made by any son, doughter or whom ever from the habibain family. At the end of the day you have to look at the object and find the clear and actually esay to see things that tell you if a signature was added or not, everything else is pure speculation.
I took the picture from the referred post and added a few things to it that will help you see what I mean.
1. Check the brown color at the arrow marked area. Why should anybody use different browns in the same design? The more rust brown is only used in the signature. If it was made in one weaving process it would be the same color.
2. Look at the vertical lines I added to the Picture. On the left side the knots lay straight to the line outside the signature but make a curve in the signature. On the right the knots are straight to the vertical line all the way. Adding or reweaving sets preasure to the foundation and this is a typical thing.
3. the light blue surrounding the writing is not anywhere else in the design.
Look at your rug, the light blue in the signature is different (lighter) from the rest also.
Finally - you are right giving not too much about the signature. Your rug is a pretty good real Nain (no copy from Naishapour) with fine wool and real silk. It has a nice design and you will enjoy the rug for a Long time.