The test

Mod A, B and C are independent of each other. I.e. they have no dependency between each other.

However, Mods B and C adds functionality to A if A is active and installed. I.e. Mod A is not a requirement but Mod B and C's adds features to A if it is active.

All this supposedly can be achieved by using Game.GetFormFromFile (Int formId, String ESPFilename).

Results

When all three are active and installed

All good. Mod A received the function calls from Mods B and C.

When A is not active and not installed

Quest B runs fine as shown with the 'questA' variable set to None.

However, Quest C simply failed to load - as shown with the error "No Papyrus Scripts attached".

Conclusion

GetFormFromFile is bugged.

The first mod that tries to access that missing Mod with GetFormFromFile is fine and works with no problems.

However, the second mod (and possibly any other) that tries to access that missing mod will always fail.

True-life case:

My mods Battle Fatigue and Injuries (BFaI) and Professions affect a third mod, Eat and Sleep (EaS) mod.

Each mod is designed to be installed, activated and to work on their own.

However, if EaS is active, then BFaI and/or Professions calls functions in Eat and Sleep.

If one of either BFaI or Profession is installed WITHOUT EaS, then that installed mod works.

However, if both BFaI AND Professions are installed WITHOUT EaS, the one or the other fails.

Of course, if EaS is installed, any combination of the two works.