I don’t think it needs to simulate the Wunderground API — if you have your own sensors to collect weather data, you can define the data format in any way suitable, and just modify the weather script accordingly. There is no need to follow the Wunderground API.
If you use solution 1, you can keep the ‘Weather Adjustment Method’ to ‘Manual’, that way the weather call will not overwrite the ‘wl’ variable, and only your script will be modifying ‘wl’ through the /co command.
Solution 2 is also fine, but if I were you I would prefer solution 1 as it’s easier to change (does not require modifying the firmware every time you make a change to the weather calculation formula).