Yes we do stand in sanchin dachi while performing our version of Tensho.
With respect to your original statements about Tensho - but applied to Sanchin:
Rooting to the ground is not like standing "on" the ground, but instead is identified as standing "in" the ground. Rooted like a tree.
Isshinryu adopted Sanchin kata as well, and we describe the sanchin dachi stance very much the same, one is to imagine one's feet are rooted to the ground, one's legs encased in sand up to the knees. One grips the ground and digs in the the toes, tucking in the tailbone and locking into the ground. This is tested with pushes and some kicks, as well as being stood on.
With this stance, are the teachings of the Tensho Kata. Old forms of this Kata we find detrimental to the health and welfare of the practitioner.
My instructor is quite unhappy with the way sanchin is currently practiced by goju-ryu, although isshinryu came partially from goju-ryu. He feels that the 'hard-soft' has become mostly all hard. We practice the goju form form sanchin, but we do not remain in tense 'dynamic tension' at all times - only at the point of locking the arms out or in.
The total body tensing and strain on the system while doing the old form of this Kata has been dropped and replaced with smoother and softer applications of the Tensho Kata's movements, as well as, larger and more rounder movements. This change is to cause the body to massage, through rotational movements, all of the joints of the body, and to do so with soft gradual change instead of hard radical change. Now older people can practice this Kata as they learn to breath correctly, and yet not overly stress their system during practice. This allows for numerous repetitions of the Tensho Kata, and also allows for the stress reduction and cardiovascular system to strengthen naturally.
We have 70+ year old karateka in my dojo, and we all do sanchin happily together. It is indeed known as the first kata taught, the last mastered. We are cautious in the way we put pressure on the knees and lower body, mindful of damaged joints and (yes, sadly) rectum from improper tenseness.
Now.... about the "Chi" phenomena.
I'm sorry if I have taken this off-track, but I do not know Tensho and I do recognize much of what you were saying from sanchin. I also have found this book very instructional:
The Way of Sanchin Kata, by Kris Wilder
In addition to breaking down the sanchin dachi stance very well, he discusses it in deep detail, including the concept of ki (chi).
Please forgive me for this sidebar comment...