How should I run this conduit for security cameras across a flat roof?
Attach to the wall and the back of the parapet. Drilling holes in the flat or sloping parts of a roof can end in tears down the line.

Consider passing the wire through the parapet instead of over it.

Conduit joins welded using the correct method, solvents and glues will not leak.

you probably don't need an expansion joint, just don't put a bracket within one foot of the corner (allow the bend to flex as the expansion joint)