Configuration
Take into account noise abatement profile: reduce thrust passing 1500 ft and start cleanup passing 3000 ft.
Take into account the relatively short runways of La Guardia: optimum take-off configuration may well have been CONF 3, which is also an approved landing configuration.
Depending on the altitude at which the (dual) engine failure occurred, configuration on touchdown may well have been the T/O configuration and may have been the ideal configuration as well for ditching.
Starting APU: yes please, if the time was available for that.
Windmill RPM: 15 % N2 might give a useable amount of hydraulic pressure thruogh the engine driven pumps of blue and green systems. Have no idea though whether engine (damage) condition and airspeed would provide even that low amount of RPM.
With regards to RAT: for sure it provides pressure to flight controls (elevator, ailerons and rudder) but cannot say from the top of my head whether it powers slats and hydraulic driven generator.
Side note: pax, as usual, at least in my media, reported engine on fire: more likely the combustion flame peeping out of both ends of the engine as a result of an engine stall due to compressor and/or turbine damage.
Looking forward to investigation results but, for now, ditching performed in an examplary manner.