I descended the Fortezza arête in summer 2003 after having climbed Piz Bernina via Spalla arete:
Starting at Fuorcla Bellavista traverse the east slopes of Bellavista to the north until you reach the track from Marco e Rosa hut / Bellavista terrace. Descend north on the glacier to the upper end of Fortezza arete. The first steps on the arete are easy. Soon enough there is a steep chimney - like chute descending on the west side of Fortezza arete (UIAA gade II, excellent rock). The route gains the now broader arete again and descends in three more “steps” mostly east of the ridge crest (UIAA grade I – II, exposed but good rock) down to the exposed and sharp ridge at the lower end of the arete.
There are fixed bolts so you easily can rappel down or belay when climbing up. There are red stripes and arrows as an additional route marker, too.
Watch out for the ice at the lower end of the arete which occurs there in late summer. There was a near fatal accident when I had a rest day on Chamanna Boval: the bloke was rescued with a helicopter after sliding and falling down several 100 meters to Vadret da Morteratsch(!).
Don´t underestimate Fortezza arete. As this is a northern exposure there often lingers snow and ice on the rocks and you may be forced to climb with crampons.
Traverse the ice or snow ridge to the upper end of little Vadret da la Fortezza. There are some huge crevasses waiting for you on the upper end of that glacier. Go straight down on the Vadret da la Fortezza to its lower end and follow the moraine talus to the Isla Pers trail junction. There are some cairns and foot tracks in the moraine scree guiding you down.
The route to Diavolezza is easy to find: just follow the good trail from Isla Pers in eastern direction down to the Vadret Pers, traverse the glacier and aim for the lower end of the zigzag trail coming down from Diavolezza hut. Zigzag up to the mountain hut and the lovely cablecar station.
The route to Chamanna da Boval nowadays is more complicated, due to the glacier retreat. Head down west on a small trail which zigzags down Isla Pers in the direction to Vadret da Morteratsch. At the upper end of the side moraine head southwest until you can easily descend the loose moraine scree and talus flanking the glacier. (On the Swiss Piz Bernina map, scale 1:25.000, map number 1277, the track goes down from Isla Pers to the confluence of Vadret da Morteratsch and Vadret Pers - due to glacier retreat that is not a good option any more !) Traverse the glacier to the other side moraine - a sometimes tricky action due to loose scree on the glacier and huge creeks full of melting water on hot summer afternoons. Fight yourself up to the moraines top where you probably will meet a trail heading north to Chamanna da Boval.
Thanks for the valuable addition, always useful to have first hand info. I've incorporated it in the route page.