Just to clarify - do you mean a Drakensberg Grand Traverse (DGT) or is this a reference to a different route such as the Great Divide Trail in Canada? I’m answering based on DGT.
Yes, propane/butane gas stoves work fine at 3000m.
I haven’t used that tent before, so I can’t comment on the specific one. But make sure your tent is able to withstand strong winds and snow - you will almost definitely get both more than once. Where possible, I recommend using caves, there are tons of them out there, and they are generally better than tents from a shelter perspective. If they are wet or the wind is coming in, you can always pitch the tent in the cave. I recently just put the inner of my tent up in a cave, which provided great shelter in a dusty windy cave.
I would suggest at least a -7C comfort rating, so an Ice Breaker or equivalent. September tends to be very cold up there.
I generally find not, but I have friends who use them. Just test them properly beforehand.
Vango make good tents. Heavier, but sturdy. And that Helvellyn is semi geodesic so that’s a good set up. Full geodesic giving most rigidity.
Google “Vango wind and rain test”, good for a laugh. Typically they’re made for Scottish conditions.