WHBM: the additional masks are every other row on alternating sides of the aircraft, more or less, if I'm remembering correctly.
Club meals are bigger/better (a bit) than economy ones. For example, the club hot breakfast gets extra bacon, a bit of white pudding, and I think a bit of tomato.
As for the leased Cello - scheduled hangar maintenance cover, nothing more. Cello's G-LENM, WDL's D-AWBA and D-AMGL, and Jota's G-SMLA are all frequent visitors to London City for various different reasons including that sort of scheduled cover.
The Skyworks flights seem popular enough on Dorniers (and Darwin's S2000s) but I don't know if they'd provide enough passengers to fill an E-Jet from the little I've seen - I'm sure someone here has numbers though!