Well, it's not really a matter of how much tinkering around you might have to do, but ask yourself this, if you do need to access the anemometer for any reason, how easy would that be stuck if it was on top of a roof!
If it were me I would definitely site the anemometer in the corner of the terrace where you have it marked on your original photo. I would attach a mast in to the corner where the two walls meet (by mast, I mean a 2" hollow, galvanized or stainless steel pipe you can get one of these easily off the web, with some kind of hinge mechanism at the base) which could be lowered to access the equipment. Depending on the height, you would probably need to secure the mast with a couple of guy wires attached to the wall. As for the height of the mast, looking at the photo, ideally it would have to be just above the height of the roof, when you need to lower it down it could be lowered entirely onto your terrace so that you can work on your anemometer.
This might all sound a bit over the top, but I assure you that as you get older you will give yourself a pat on the back for going down this route.
Whether you go for my suggested mast option or a shorter fixed one it's up to you, but if it were me I would definitely keep off that roof.
PANDY weather.