It looks like you are asking several questions.
1 What happens when a kite is flown in wind speeds above its recommended range?
Theoretically, a perfectly balanced kite in smooth, ever increasing wind, will rise and bow more until it breaks. However, if the spars or sail are not symmetric, the kite will veer to the weaker and/or heavier side.
2 How do I make a T-Delta behave in very turbulent winds?
In my limited SLK thinking, I would add a floaty/long tail to the very bottom of the spine. This should re-orient the spine along whatever the current wind condition is and tip the entire kite to a higher angle of attack, which should reduce the apparent sail area. You could also shift the tow point to reduce the AOA. I have a 100' x 7" dia tube tail which I use on my Roks when the wind gets feisty.
3 Should I be concerned about fabric stretch?
If the fabric is Nylon, it should stretch and recover more than the modern fabrics, which are much stiffer. In material terms we would say Nylon is ductile and Icarex (e.g.) is brittle. Think of the way in which a grocery bag fails whether it is a plastic (ductile) or paper (brittle) bag.
There are many people here more versed on the subject. I'm sure they will chirp in soon.
Welcome to the Forum.