Literally unplayable!

Jokes aside, it's not just you, they used the wrong symbol.

So I've noticd a currency we can't get yet, the Mercury points. My question is, is that a premium currency ONLY buyable with real money ?

Welcome.
AFAIK yes, Mercury points are only obtainable with real money.

If so, what will be exclusively limited to Mercury points ? Skins, or maybe additional characters ?

Skins mainly, but you can unlock characters with it as well if you so desire. Characters are also unlockable with gold, which is the in-game currency you can earn by playing.

Also the mission screen states 2017 season. So far we can assume this means there will be a new season of story every years as long as the game is active. However will these be paid ? (considering the ticket currency my guess would be that, unless missions need to be unlocked later, which would make sense considering it's a story with an order ; a beginning and an end)

Yes, new seasons will be paid.
(I'm not exactly sure if campaigns can be bought separately or not).

However, the ticket system is such, that if you own a campaign and buy a ticket, you can send it to your buddy who owns the game, but doesn't own that particular campaign, and it let's him/her join you on a mission once.

Had this happen to me before, rubberbanding and slowmo-ing because of a bad host is the worst experience ever. They absolutely need to do something about this. Even having an opt-out option for hosting would be nice as well.

Yes, antag needs more green FX.

What copnfused me was that whilst initially it seemed the AI could not see me if I let my aggro bar reduce, later on that bar seemed to be constantly full. Some mechanic I am not getting going on there.

That is because different factions have different stats. Those stats are similar for characters in the same faction. As an example, for Umbra Wardogs (Alicia), the aleph bar is constantly at maximum or The Fifth Council (Konstantin, Dr. Kuzmann) are slow and tanky.
Although I've suggested to the devs that this should be balanced per character, not per faction, because it makes more sense to fine-tune stats for a character with it's own loadout and let people have different playstyles in each faction, so if someone likes the aesthetic of a given faction, they can still find a character that suits their playstyle.

Yes, why such few invitations?  I've been signed up awhile.. I get emails, no key.

Also, is there no NDA for this game? I figure there isn't because this Beta forum is visible to everyone, even people without an account.

AFAIK getting emails about the beta means you're in, you may have missed out on how to get your key. You'll have to ask the devs for it.

That is right, there's no NDA.

I really like what Elyleo is suggesting, very clean HUD, good work!