Is it possible for the titanium screws used in a spinal fusion to break?

Yes. Screws fail from cylical loading like nending a hanger untilit breaks all metal implants can break in this way a sign of persistant micro motion non fusion usually screw breakage is low energy unnoticed event seen on follow up xrays.
Yes. The screws can break although not frequently. Often times, it is due to a failed fusion (pseudoarthrosis).
Yes. If there are high enough loads or stresses across the screws they can break. This is more often seen with fusions not healing or with significant loads placed on the screws such with major deformity corrections and in large, overweight patients. Sometimes, the screws are undersized or of an earlier design.