so, Ive been using the wind overlay for planning (as per this topic), and its made things SO much better … really happy now 
ofc, there are bugs, and its a bit of a hassle to actually find the mission you want to do… as really the tooltip should be giving us this ‘overview’ - but at least its works well enough, am Im pretty confident in using it to plan.
I still haven’t 100% decided if the arrival weather is determined using take-off time or if as you progress the flight, its moving thru the historical data (up to live point).
I get the feeling its progressing as you move thru the flight - but Ive not yet had a situation where the weather change enough to know for sure.
one example, Ive seen (but is not conclusive) is to do with conditions (vfr/ifr)
I had a mission that was within 1 an hour of live time, but would take 3 hours to fly.
it said departure VFR, arrival UNKN
my assumption is because during the flight you are moving into live weather, so it cannot know the weather for arrive at take off time.
note: if weather is set at departure time then arrival would be known !
BUT…
because we have 24 hour of historical data, on this flight I could actually move the departure to use the weather from 24 hours previous. (as we have an hour window on flights),
doing this , I got departure VFR, arrival MVFR (rather than UNKN)
I speculate this is because its now using the historical weather of -24 hours to -21 hour
so in theory this is ‘proof’ weather time is moving as we fly
but why is this not 100% proof?
because Ive seen quirks in the past where I get UNKN when the weather should be known.
i.e. I cannot 100% say, unknown is because its only due to live.
that said, Ive seen this a few times, so was not a one off``
so for now, its still just feeling this is what is happening…
my working assumption of how its working… and what Im trying to test 
at take off, it uses the mission start time (which we know we have an hour window on)
then during the flight it progresses thru historical data using sim time for your location (in flight)
if you use sim rate (or you start at live time), then at some point it’ll run out of historical data, and will just move to using live weather.
note: weather is reported infrequently, and only at certain locations - so Id assume its using interpolation between locations and also time slices.
ofc, this could be ‘wishful thinking’ , but given we know the sim has ‘live weather’, and we also know it has historical data - it doesn’t seem too far fetched/complicated to work this way. its just where you source your data points from.
Id also assume, free flight works exactly the same as career - I dont see any reason for them to differ. I think career missions essentially just set parameters for a flight.