As the bracket "only" has a threaded rod, only one unit can be mounted per bracket. Ergo, one bracket is needed for each unit.

I am of the opinion that this should work. Diameters up to approx. 10 cm are possible with the straps we have.

Yes, this should work. I have attached the sensor to a horizontal post and the rain sensor to a vertical one. Thanks to the ball head, you can then align the respective sensor, so this should actually also be possible with an inclined one. (My two posts are also not in balance horizontally and vertically).