A. To kill or damage plants, Roundup must be sprayed onto the green parts of the plant. Older parts of the plant which are not green or no longer green won’t absorb any. If your gardeners sprayed the ground surrounding the Saguaro there should be no problem for the Saguaro.
There is an exception. Roundup can move in water and cause damage. Never spray Roundup where there is moving water.
Any pesticide including Roundup can move with wind or significant air movement. Never spray when there is air movement over 2 mph. never spray the hot surface of a soil or rock because pesticides can volatilize, move upwards with warm air and damage plants.
As long as landscapers or applicators are careful around plants there should be no problems.
In my opinion, the reason your Saguaro started leaning is because of how it’s irrigated. Saguaros support themselves with roots that extend long distances from the plant. This stabilizes it as it gets taller.
If irrigation is applied close to the trunk and there is no other water available then roots will not grow very far. As it gets larger, it gets top-heavy and these short routes can no longer keep it upright.
What to do? I sent you a picture of how to properly support a Saguaro after it has been planted and until I can get established. Install other plants with drip irrigation at several distances away from the Saguaro. If the soil is wet, the roots of the Saguaro will find this water and grow further from the trunk.