Complex sentence: Sam tells Quinn that David Weintraub has been in touch and Quinn agrees to meet him, but it turns out to be Irving Moss accompanied by his former protégé and fellow sadist Duncan McCrea, who turns out to have personally blown up Simon with the belt bomb.
Split into simpler sentences: Sam tells Quinn that David Weintraub has been in touch, and Quinn agrees to meet him. The'David Weintraub' who appears turns out to be Irving Moss impersonating him, accompanied by former protégé and fellow sadist Duncan McCrea, who turns out to have personally blown up Simon with the belt bomb.