I was once taught that sometimes we can bear our testimony in silence... how? by doing as you said above... you actually invite the spirit and that the spirit can testify to her in a way your words can not.  Testimonies are not always vocal...sometimes they are done in silence. 

But never pass the chance to even give meager words because the spirit has the ability to make those humble words earth shattering.

you could always write it down on a piece of paper, put it inside a balloon and hand her the balloon.  She will never forget your testimony, I bet !  Just my suggestion.

all too often as others are giving their testamony, i pray to HF and have all kinds of things to say. but when i get up, i have very little that comes out. some of it may be nerves, some of it is like a switch that gets turned off. i have tried to look at my motives for giving a testamony and don't think they are false ones (motives).

when i was a stake missionary, my testamony came out nicely and without hinderance.so i dont get it, but there it is, and you know what? i like the prayers i feel during testamony time, so for now, at least, thats where i'll stay and what i'll do.

for me hearing other's testimonies helps remind me of my own.  i'm a forgetful person, the lord has tought me lessons but i need regular reminders of what i know.  even a fumbled testimony can carry the spirit, can make me think, "i know that too...." and then all about the experiance that tought me that. 

your last paragraph is perfect.  concise to the point are sometimes the most refreshing and powerful testimonies.

My suggestion would be to stand up and let the spirit guide you. Every time I bear my testimony it is with reluctance. But then the spirit will fill me and my feet start moving, before i know it im standing in front of all, feeling as if im rambling. Then it never fails that someone will tell me that my testimony touched them in one way or the other. Then some months, nothing. I feel no desire at all to speak. 

There is a man in our ward who bears his testimony every fast sunday. I enjoy hearing him so very much. He is an older man and it always seems at first as if he is just rambling about nothing signifigant. By the end of his stories though he always makes a point and provokes all of us to think about ways we could be more christlike.

It is my thought than when the spirit moves us it is because someone in that room needs to hear it. We may never know who or why but Heavenly Father does, and he uses the spirit to guide us there.