It all depends on too many factors, so let me offer another view, not mentioned yet:
- There is another inteligent, skilled and ethical rase Threasan living on Thrae just couple of LY from Earth.
- They live long, so they can wait for light-slow communication. The problem is, they use some other unknown to us kind, not radio, so they did not hear us, we did not hear them, even when our respective signals reach the other party long time ago, it was not noticed.
- They have vaguelly similar biology as we have, so Earth was considered "habitable, but probably unhabitated" planet by them (and vice versa)
- FTL travel is possible, but it is way expensive to enter "hyperspace" and moreover it is more like firing gun - nearly all energy is spend on the start and the fuel undergo some kind of chain reaction affecting all fuel in chain-range like thousand miles around - so it is one way extra expensive ticket now, but they can get a lot of baggage with them on the fly for not so big additional cost.
- There is no exact evidence, that Earth is really habitable and usable for them
- Sending lot of automated sonds would be too expensive (in planetary range too expensive) and their Artifical Inteligence is not as good as their natural inteligence (especially in unexpected conditions)
- But they can send, say, about 10 Threasan (of both sex) and like year supply of food, air and such. And there is like thousands volunteers to one-way ticket (on the Earth would be for sure), willing to go even if they would die, but they would be HEROES and STARS, not mentioning all kind of possible scientific gain, they can collect there.
- There is even chance, that the Earth would be habitable, in which case they can survive lot of years and wait for next colonist and establish "new Threa" here (and live to their natural death seeing their grandchilds colonise new word)
- Also there is chance, that Earth would have a lot of unobtainium, so Thrae would send extra big and extra expensive second ship which could bring complete fuel factory and separated materials to compose fuel for way back and establish "both-way trade route"
- and third, it is even possible, that the special fueld can be found on Earth in easily collectable (and processible) form, so they could return on their own eventually.
So one day with sound BLIP the Threasan ship pops out somewhere near Earth orbit (or not too far), with 10 aliens on board. They realise, that there is a lot of satelites, cities visible from orbit and such. They even found, that we try to comunicate, and while our radio technologies differs, we can use light reflections, pictures on ground (Nazca), or maybe lasers (not to cut them, but to direct light only on their ship), communication is somehow established. And now there is a problem what to do:
- They can just orbit Earth for year, comunicate and heroically die, maybe even ask us to help direct their ship (and our support ships too) into the Sun, to evade possible contamination. Nice, but they will die near big source of fuel (say salt water, 1 cubic mile needed) in their reach (quarantine==infinity).
- They can land somewhere anyway, hope to be able somehow work out secure contact and do not destroy any race. (And now it is on us, if we would cooperate, or if we just nuke them as invaders - and their next ship too)
- Thay can colonize moon instead, we would supply them with some food, air, etc to sustain them there for life long, even helping them establish colony there. Our scientist (well morsels first) would travel (one-way ticket for start) there and make another colony near them, with third colony in between.
For start the third colony would be one-way-in for both parties, they would send their food and their morsels the, we would send ours and if both morsels survive for a month both parties coud send more food and one or two volunteers. If even they survive and be seen as healthy by both ours and theirs medical technology (and we can microscope all kinds of microorganisms their for both parties knowledge), then more volunteers can go in and mix both technologies and materials and food and such, in relatively safe way.
After couple of months of human/Threasans living in third colony without any (unsolvable) problems. As humans food (or other sources) shows to be eatable by Threasans, they can survive for life long, joining third colony and theirs (so far uncontamined) for more space and comfort at (small) risc of long term infection. Also more screw for our colony and more volunteers for co-living. Eventually our colony (still uncontamined) would split to part, which would return to Earth and part, which would join common living space.
With our resourses and their knowledge and preparations there would be small long term colony living on Moon.
After some time (and I would think about two or four years) when no ubeatable infection would detected by any medical technology it would probably be safe to create colony on Earth, in something like Area 52 (or Section 9) as well as create similat colony establishment on Noom orbiting Thrae. Their ship would refuel, (and as we know, that their food is basically eatable by humans too,) then without reserves for a year, but only for a days there can stuck a lot of humans and some Threasans for heroic return to their Noom colony to continue on discovering our mutual compatibility (now hosted by Thrae).
If after 20 years nobody in Area52 is hard alien/mutation-infected, it is probabely safe, as it means that we can live together long to have childs, living long to have childs and so on.
Also if there are some problems from start, we can work together on vaccination for that, as there are already both parties, one of them allready immune and having developed resistance, so there are many ways, how we can together fight the bacteria and make the affected party immunised too (or at least found how to suppress the spread of the bacteria).
If any deadly infection is detected in the way, that nuking Moon or even Area52 is still possible, but the longer, the more it would be probable, that nobody dies from infection, so even if there would be some, which would target some untested population (say those with some unusual genes combination), then there would be also big part population naturally imune (or able to be vaccinated against) such infection.
Well that is based on good will and cooperation of majority of Earth population, so it is sci-fi after all probably, but at least it could work there.
The main point is, that Thraesans, while having FTL are not having it so much to be able effectively overhelm us near Earth and humans not having FTL are not able overhelm them at their home.
On the other hand being in close contact with inteligent beings, just on Moon (or later on Noom) would lift a lot burdens of long time quarantine, making it only long time work at distant place, but still relatively free and surely not alone.