What would be the most obvious changes that could be observed once the ET Alliance leaves? I assume the weather would drastically change for the better immediately, i.e. no hurricanes anymore, no earthquakes, etc. Could we expect it to play out that way or would it be less obvious for some reason?

This indeed will be observable. There may be some dark doings as a carryover by the members of the Mercenary Army Program. It is uncertain what their fate might be if there should be a withdrawal in the future. There are many trade-offs involved in changing history. Much depends on the nature of the manipulations and the underlying factors that can be done in the past to prepare for a change in the present. We are endeavoring to serve all. That may well be an impossibility. They consider the MAP to be personal property. They will not want to leave any secrets behind that could be discoverable, so it is one thing to get them to withdraw and it is another to create increased risks for them in other respects. They may wish to erase any traces to their knowledge base and technological capabilities so they may endeavor to remove any infrastructure, human or material, that could be exploited against them, if in the future they change their plans and return. There may be dark doings in the planning that have been ordered to take place, which they may simply allow to continue. The withdrawal may happen in stages, so the effects will be perceptible more in stages than with a sudden shift where everything is now neutral or positive. The corruption of humans will not be undone by them, but only through prolonged healing efforts. The corruption of individuals to have many dark beliefs that are faulty and will not serve them, will not go away on its own, so people who have become corrupted will remain corrupted, it is just they will not be given new marching orders from the interlopers. It is possible that the extraterrestrials will withdraw before the dark spirits are dealt with, so that will require a concerted effort to continue the work of healing in earnest to remove that menace to a full extent. At the moment, the energies are too uncertain to give you a clear picture let alone a timetable for the sequence of events and consequences that will play out, but any lessening of difficulties will be a blessing and without the wholesale annihilation of humanity looming, everything else will not only seem more manageable but be more manageable.