I'll try to explain again.
The rules require that the aircraft be a maximum of xxx minutes from either departure, destination or alternate.
This time is a statistical based figure. It can be longer if one can comply with certain requirements / have reliable equipment and procedures etc.
Having set the time with the authority, then in order to draw some circles on a map, a speed must be used. Clearly the higher the speed then the bigger the circles. The rules limit the maximum speed that can be used. The speed used must be shown in the ops manual.
So we have our chart with some circles. The position of the aircraft can never be at a place that is outside of all the circles drawn. i.e. one can end up with no-fly zones.
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When it comes to the pratical operation of the flight then the flight will be planned based on not just the winds and OAT but also the payload and MEL issues. The route will never be through the no fly zone mentioned above. However, the enroute alternates will be selected to provide for the aircraft to arrive at the alternate in the expected conditions with the required fuel reserves when the aircraft is diverted from the ETP.
There will be several ETPs and the operational flight plan will show how the fuel is calculated when flying from the ETP to each of the alternates. This will include winds, level, TAS/Mn and usually fuelflow/eng etc.
If conditions are as expected then fly as per the operational flight plan and you will (should) arrive at the alternate with the expected fuel reserves.
If conditions are different then make adjustments to arrive at the alternate safely or fly to a different alternate (the ETP may have moved if the conditions are different).
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So I hope that you can see that there is the statistical calculation that determines if ther are some no-fly zones / the size of those zones. Separate to that is the practical operation of the flight which is no different from any other flight with enroute alternates.