I will be assuming that problems such as the common cold killing these invaders does not apply.
I actually think you're giving the invaders too many handicaps.
2 out of 5 ships don't make it?!?! That's a ridiculously high probability of death. How in God's name would you convince anyone to board the ships? Who would want to be in the navy?
I understand having to use couriers, but with the above likelihood of them actually delivering their messages your communications will be insanely poor.
Those mother-ships actually contain very few troops and armaments. Consider that a modern super-carrier has about as much crew as you do soldiers, and carries a LOT more planes. Also, funny enough, the city of New York has 34, 000 uniformed officers. You would need 3 full mother-ships to even bring that many soldiers to Earth.
Your invaders have zero chances of winning a conventional war. They will attack, win some major battles, definitely change the planet's political landscape forever, but ultimately they will lose.
Nukes in Space
Humanity can and will use nuclear weapons against these invaders. Some think that the US has already weaponized satellites, even though they've signed treaties to the contrary.
On your own world that might not have happened (weaponized satellites), but believe me when I say that humanity will make it a top priority to get a nuke up to the mother-ships once the hostilities are underway, and you will have a very difficult time stopping them. Remember "Independence Day"? Nothing quite so dramatic would be necessary:
Launch a "smart" missile into orbit from one side of the world where you're not looking (we have nuclear submarines that can launch at a moment's notice from all around the planet). Have it sit in orbit, among all the other junk up there until it drifts closer to your ships, then activate/home in when it gets close enough. Eventually one will inflict massive damage
Nukes on Land
Due to the low number of troops available to them, the invaders will need to concentrate at only a limited number of landing sites. This will make them incredibly vulnerable to a tactical nuclear strike.
Look at how expensive in $$$, manpower, and equipment the Iraq/Afghanistan "conflicts" have been to the US military. A homemade bomb buried on the side of the road can take out a main battle tank worth millions of dollars, and its highly trained crew (because, surprise, tanks haven't historically needed much armoring underneath before).
Your force might successfully defeat the main military forces of the invaded world, yet be destroyed one vehicle at a time by the survivors in only a few short years.
Approaching the Planet
Your objective is to take the planet while not contaminating the land mass, or, of course, dying. What you need to do is to absolutely overwhelm the indigenous population before they have a chance to organize, and react to your particular vulnerabilities.
Stealth is Preferable
As you approach the planet try to remain undetected. The later humanity finds out you're coming, the better.
You should be monitoring their communications. Is there talk of strange objects moving through the solar system on the news? Has encrypted traffic through military satellites suddenly spiked? You need to know.
If detected initiate a conversation. Deceive the silly humans with talk of being peaceful galactic explorers in search of new civilizations. You are ambassadors of your gentle and friendly species, bla bla bla.
While military leaders may well be suspicious, the liberal masses - brainwashed by years of propaganda - will demand that the governments extend you a friendly welcome. The military will be crippled in their ability to organize against your arrival (although they will be on alert)
Kill the Earthlings
The only way you're taking the planet is by killing so many humans that their civilization crumbles, and their ability to mount any sort of organized attack is utterly defeated.
Furthermore, the more devastating your initial attack, the less likely you are to have to deal with pesky survivors planting road side bombs, and sniping your troops.
Drop the Hammer
Drop rocks on every major population center in the world. Humanity has no good way of stopping you. Some might have time to evacuate - such as the leaders, to some underground bunkers, but your troops can target those. However, the only way of stopping Billy-Bob and his cousin Bobby-Ray from playing IED tag with you in the foothills of some southern state is to make sure that they never have a chance to get to their pick up trucks and drive up into the hills in the first place.
Even as you are convincing the silly Earthlings of your "good intentions", your ships should be rerouting asteroids from the orbits of other planets onto the capitals and populations centers of Earth. As those are dropping, so should your own fighting forces be bombing remote military bases, targeting carriers at sea, and hunting submarines (if you have a way of finding them).
It's worth noting that dropping asteroids on population centers does more than just the obvious (squishing icky humans). These will cause raging fires which will sweep across the land. You can start massive forest fires which will ravage the countryside unchecked, as response crews are either dead, or busy elsewhere.
Furthermore you will be destroying food production (farms burned by forest fires, etc), as well as transportation arteries (highways and train tracks). With food suddenly becoming scarce, and with no means to transport what they do have, the survivors will face starvation on top of fire fighting, and conducting rescue operations.
If you have the means of stealthily approaching the planet then you could potentially end the war in just a few short months, and not even have to fire a shot to do it.
By doing just a little bit of research on humanity's pathetically unshielded internet you will discover that there are some research centers on their world which contain incredibly dangerous viruses and diseases (some have even been weaponized by the stupid bipeds themselves!).
Have a few elite squads storm one of these complexes and gather samples. Humanity won't even know what hit them. By the time they get over their disbelief of an alien invasion they will already be dying by the tens of thousands.
With your ability to spread the virus around the entire planet in only days (have ships rain containers of the virus over every major population center) humanity will have no way of fighting the sudden outbreaks. Some will survive by fleeing to remote areas, but at that point they become easy targets to a few well placed asteroids.
This method is especially effective, as any time guerrilla troops are operating in an area all you need to do is release more of the virus around their general location.
With humanity crippled (and dying in droves) by either disease, or orbital strikes (preferably both) concentrate your military might in a few areas. Your troops and tanks can move through the desolated areas with impunity, and hunt down survivors using the most brutal methods you can think of. Incidentally, have you heard of air-fuel bombs? If you haven't before, give them a whirl. The things those silly humans can come up with!
Your main threat at this point will be nuclear attacks launched by ballistic submarines hiding in the oceans. With a little care, and the ability to intercept missiles in the high atmosphere, however, you should be fine.
And so, could an interstellar war be remunerative from an economic point of view? Yes - Especially if you take the biological warfare approach.
With humans dying of some exotic disease you can simply walk in and take whatever you want. Their art, if your people are into that. All the gold and other precious/rare metals from their vaults, and factories. The list goes on.
NOTE: Take a look at John Ringo's Live Free or Die novel. In it he deals with an invading alien force (which is pathetic by intergalactic standards) absolutely dominating Earth's military, and basically holding the entire world hostage, while demanding all our precious metals (which we mine for them). How do they do it? By first dropping a rock on some cities, then telling us more will follow unless we comply. Governments around the world will comply.
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