To answer the 1st ? about FCA's dual-callsigns, the four digit alpha-numeric codes, eg FCA112c are our ATC callsigns, whilst the 4 digit numeric codes, eg FCA4616 are our commercial callsigns, so what the pax are ticketed for and whats displayed in the terminals. They usually bear no relation to one-another.
As littlepilot noticed, this only applies to EU flights - non-eu flights have the same atc/commercial callsign. For those interested, the commercial callsign displays info about the flight itself. The first no, in the above example '4' denotes the home-airport, in this case LGW (2 - MAN, 3 - BHX, 1 - GLA, 7 - BRS,5 - LTN/STN, 6 - Adhoc flight, 8 - BFS/DUB), the second number shows day of the week, so '6' represents a saturday (monday -1, tuesday -2, etc). The remaining numbers specify the flight.
In response to the above ? from Hatters Utd about fuel stops whilst on-route to Puerto Vallarta, the LGW one needs it due to the slight extra distance it has to fly - the MAN one doesn't need the extra stop (taking into account contingency fuel etc), but is near the limits. The extra few hundred miles makes all the difference!
PS - Glad you liked the extra legroom!