Well, you did send in a memo so I might as well heed the call of the wild to check upon it...

So we are dealing with a hypothetical situation, which we are already grounded upon a metaphysical arrival in a tension-wide World. I like that type of situation; yet let's get to the first set of questions.

1. I only know one religion (two if you count Neo-Confucianism as a religion) group that would basically modify but remain the same in content and form: Buddhism. The Abrahamic religions will be fractured six ways from Sunday, at least the fundamentalist strains. (Islam, particularly more for Sunni and less for Shi'iate Islam, will have a funny course of modifications; yet they are part of the Abrahamic religions and shall be modified nonetheless).
2. Particularly me: I couldn't care less if a God Alien or a normal LMAO green small alien came up to me or to the doorsteps of the World's Capitals. I accept that they could exist but we have yet to make contact to know each other's existence truly. So the question's answer would be: indifferent.
3. Just look above: I am indifferent to them existing or not existing.

Now this is an interesting set of statements, one from an existentialist line (more Jean-Paul Sartre), one to talking about the demystification of Science which allowed Science to start demystifying the World, one of the extraordinary claim people (who need extraordinary evidence) and one about us not truly knowing if we can validate these theories truly. BUT MORE QUESTIONS!!!! FEED ME MOAR!!!!~ :p

1. My only answer is: will it matter? Never mind that souls take on different contexts under different religions, but that souls are highly debated even in theological spaces. Them being here will definitely through a monkey wrench to the discussion and we'd have to make new categories for theology and the philosophy of religion to figure out.
2. Let the theologians and philosophers banter that about; but they are already figured it to be an automatic yes by the inclusion of them being in a different plane of reality whenever they left the Earth.
3. Religiously, I only leave that to a God to decide. But I care not for a God to exist; if God Godself existed, cool I shall shake the hand of God; if God Godself doesn't, then into the empty Void shall my carcass go.
4. That's going into bounds of metaphysicality that the Second World Theory bitched on about with water and its molecular compound being possibly similar to a possible molecular composition on Planet X with molecules XYZ being the Planet X equivalent to H20. However, this is the bounds of metaphysical thinking that needs a fleshing out a rigourous dialectical exploration.

Yay, stats! I love 'em! And it is possible that oral history had been manipulated by writers seeking out their brand of a written religion; this is most certainly true when it comes to the Bible and Emperor Constantine censoring and making propaganda of heretics of those that had written those parts of the Bibile in. So it only is the case that alien civilizations decided to take a small pitstop, which turned into oral history which later got written down and twisted by society until we get to today where you are asking the question of them possibly being here. Infinite possibilities in the metaphysical World. Anyways, MOAR QUESTIONS!!!!~ :p

1. I already gave thoughts above on that possibility. So I shall use this to talk about this post being a good hypothetical post but also being a conversation starter post. I like it, good for "coffee" talk - despite me despising coffee.
2. Well if the material conditions betters, we can see a plummet on religious people; yet institutional religion will be a tough nut to crack and will have to be in the hands of the laymen before they disappear. And this will be especially true if ETs don't decide to crash into our planet and waddle on it!~

I shall share my opinions again: it would be cool to see them but I shall remain indifferent to the arrival of aliens; for sure it will change the Superstructure (Culture and Ideology) of every society and it might have a rammifying affect on the Economy due to how people take the event.

On the topic of religions, I do like what carnedes.org is doing right now when he goes and explain the philosophical terrain of religions.